Added some documentation for RelNotes. More will follow shortly.

doc/sql.extensions/README.context_variables

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+------------------------
+System context variables
+------------------------
+
+
+CURRENT_CONNECTION / CURRENT_TRANSACTION (FB 1.5)
+-------------------------------------------------
+
+  Function:
+    Returns system identifier of the active connection/transaction,
+    i.e. a connection/transaction, in which context the given SQL
+    statement is executed.
+
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    CURRENT_CONNECTION / CURRENT_TRANSACTION
+
+  Type:
+    INTEGER
+
+  Scope:
+    DSQL, PSQL
+
+  Example(s):
+    1. SELECT CURRENT_CONNECTION FROM RDB$DATABASE;
+    2. NEW.CONN_ID = CURRENT_TRANSACTION;
+    3. EXECUTE PROCEDURE P_LOGIN(CURRENT_CONNECTION);
+
+
+ROW_COUNT (FB 1.5)
+------------------
+
+  Function:
+    Returns number of rows, affected by the last SQL statement.
+
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    ROW_COUNT
+
+  Type:
+    INTEGER
+
+  Scope:
+    PSQL, context of the given procedure/trigger.
+
+  Example(s):
+    UPDATE TABLE1 SET FIELD1 = 0 WHERE ID = :ID;
+    IF (ROW_COUNT = 0) THEN
+      INSERT INTO TABLE1 (ID, FIELD1) VALUES (:ID, 0);
+
+  Note(s):
+    After SELECT statements ROW_COUNT always contains zero,
+    i.e. it can be used for INSERT/UPDATE/DELETE statements only.
+    It may be a subject of change in the following versions.
+
+
+SQLCODE / GDSCODE (FB 1.5)
+--------------------------
+
+  Function:
+    Returns numeric error code for the active exception.
+
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    SQLCODE / GDSCODE
+
+  Type:
+    INTEGER
+
+  Scope:
+    PSQL, context of the exception handling block.
+
+  Example(s):
+    BEGIN
+      ...
+    WHEN SQLCODE -802 THEN
+      EXCEPTION E_EXCEPTION_1;
+    WHEN SQLCODE -803 THEN
+      EXCEPTION E_EXCEPTION_2;
+    WHEN ANY DO
+      EXECUTE PROCEDURE P_ANY_EXCEPTION(SQLCODE);
+    END
+
+  Note(s):
+    1. GDSCODE variable returns a numeric representation of the
+       appropriate Firebird error code.
+    2. Both SQLCODE and GDSCODE always evaluate to zero outside
+       the exception handling block.
+    3. If you catch exceptions with 'WHEN SQLCODE' block, then only
+       SQLCODE variable contains the error code inside this block,
+       whilst GDSCODE contains zero. Obviously, this situation is
+       opposite for 'WHEN GDSCODE' block.
+    4. For 'WHEN ANY' block, the error code is set in SQLCODE
+       variable only.
+    5. If user-defined exception is thrown, both SQLCODE and GDSCODE
+       variables contain zero, regardless of the exception handling
+       block type.
+
+  See also:
+    README.exception_handling
+
+
+INSERTING / UPDATING / DELETING (FB 1.5)
+----------------------------------------
+
+  Function:
+    Determines type of row operation being executed.
+
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    INSERTING / UPDATING / DELETING
+
+  Type:
+    BOOLEAN (emulated via pseudo-expression in FB 1.5)
+
+  Scope:
+    PSQL, triggers only.
+
+  Example(s):
+    IF (INSERTING OR DELETING) THEN
+      NEW.ID = GEN_ID(G_GENERATOR_1, 1);
+
+  See also:
+    README.universal_triggers

doc/sql.extensions/README.data_types

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+---------------------
+Native SQL data types
+---------------------
+
+
+BIGINT (FB 1.5)
+--------------
+
+  Function:
+    SQL99-compliant exact numeric type.
+	
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+	
+  Syntax rules:
+    BIGINT
+	
+  Storage:
+    64-bit, signed
+	
+  Example(s):
+    1. DECLARE VARIABLE VAR1 BIGINT;
+    2. CREATE TABLE TABLE1 (FIELD1 BIGINT);
+	
+  Note(s):
+	Quote from the SQL-99 specification:
+
+	SMALLINT, INTEGER, and BIGINT specify the data type exact numeric,
+	with scale of 0 (zero) and binary or decimal precision. The choice
+	of binary versus decimal precision is implementation-defined, but
+	the same radix shall be chosen for all three data types. The precision
+	of SMALLINT shall be less than or equal to the precision of INTEGER,
+	and the precision of BIGINT shall be greater than or equal to the
+	precision of INTEGER.

doc/sql.extensions/README.exception_handling

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+------------------
+Exception handling
+------------------
+
+The common syntax rules for EXCEPTION statement is:
+
+  EXCEPTION [[name] [value]];
+
+
+Run-time exception messages (FB 1.5)
+------------------------------------
+
+  Function:
+    Allows to throw exceptions with text message
+    defined at runtime.
+
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    EXCEPTION <exception_name> <message_value>;
+
+  Scope:
+    PSQL
+
+  Example(s):
+    1. EXCEPTION E_EXCEPTION_1 'Error!';
+    2. EXCEPTION E_EXCEPTION_2 'Wrong type for record with ID=' || new.ID;
+
+
+Exception re-raise semantics (FB 1.5)
+-------------------------------------
+
+  Function:
+    Allows to re-initiate catched exception.
+
+  Author:
+    Digitman <digitman@hotbox.ru>
+
+  Syntax rules:
+    EXCEPTION;
+
+  Scope:
+    PSQL, context of the exception handling block
+
+  Example(s):
+    BEGIN
+      ...
+    WHEN SQLCODE -802 THEN
+      EXCEPTION E_ARITH_EXCEPT;
+    WHEN SQLCODE -802 THEN
+      EXCEPTION E_KEY_VIOLATION;
+    WHEN ANY THEN
+      EXCEPTION;
+    END
+
+  Note(s):
+    Evaluates to no-op if used outside the exception handling block.
+
+
+Run-time error codes (FB 1.5)
+-----------------------------
+
+  Function:
+    Allows to get a numeric error code for the catched exception.
+
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    SQLCODE / GDSCODE;
+
+  Scope:
+    PSQL, context of the exception handling block
+
+  See also:
+    README.context_variables

doc/sql.extensions/README.universal_triggers

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+---------------------------
+Universal triggers (FB 1.5)
+---------------------------
+
+  Function:
+    Allows triggers that handle multiple row operations.
+
+  Author:
+    Dmitry Yemanov <yemanov@yandex.ru>
+
+  Syntax rules:
+    CREATE TRIGGER name FOR table
+      [ACTIVE | INACTIVE]
+      {BEFORE | AFTER} <multiple_action>
+      [POSITION number]
+    AS trigger_body
+    
+    <multiple_action> ::= <single_action> [OR <single_action> [OR <single_action>]]
+    <single_action> ::= {INSERT | UPDATE | DELETE}
+
+  Example(s):
+    1. CREATE TRIGGER TRIGGER1 FOR TABLE1 BEFORE INSERT OR UPDATE AS ...;
+    2. CREATE TRIGGER TRIGGER2 FOR TABLE2 AFTER INSERT OR UPDATE OR DELETE AS ...;
+
+  ODS:
+    Encoding of field RDB$TRIGGER_TYPE (relation RDB$TRIGGERS) has been extended
+    to allow complex trigger actions. Coding scheme is shown below (in C syntax):
+    
+    // trigger type prefixes
+    #define TRIGGER_BEFORE		0
+    #define TRIGGER_AFTER		1
+
+    // trigger type suffixes
+    #define TRIGGER_INSERT		1
+    #define TRIGGER_UPDATE		2
+    #define TRIGGER_DELETE		3
+
+    // that's how trigger action types are encoded
+	/*
+    bit 0 = TRIGGER_BEFORE/TRIGGER_AFTER flag,
+    bits 1-2 = TRIGGER_INSERT/TRIGGER_UPDATE/TRIGGER_DELETE (slot #1),
+    bits 3-4 = TRIGGER_INSERT/TRIGGER_UPDATE/TRIGGER_DELETE (slot #2),
+    bits 5-6 = TRIGGER_INSERT/TRIGGER_UPDATE/TRIGGER_DELETE (slot #3),
+    and finally the above calculated value is decremented
+
+    example #1:
+        TRIGGER_AFTER_DELETE =
+        = ((TRIGGER_DELETE << 1) | TRIGGER_AFTER) - 1 =
+        = ((3 << 1) | 1) - 1 =
+        = 0x00000110 (6)
+
+    example #2:
+        TRIGGER_BEFORE_INSERT_UPDATE =
+        = ((TRIGGER_UPDATE << 3) | (TRIGGER_INSERT << 1) | TRIGGER_BEFORE) - 1 =
+        = ((2 << 3) | (1 << 1) | 0) - 1 =
+        = 0x00010001 (17)
+
+    example #3:
+        TRIGGER_AFTER_UPDATE_DELETE_INSERT =
+        = ((TRIGGER_INSERT << 5) | (TRIGGER_DELETE << 3) | (TRIGGER_UPDATE << 1) | TRIGGER_AFTER) - 1 =
+        = ((1 << 5) | (3 << 3) | (2 << 1) | 1) - 1 =
+        = 0x00111100 (60)
+    */
+
+  Note(s):
+    1. One-action triggers are fully compatible at ODS level with FB 1.0.
+    2. RDB$TRIGGER_TYPE encoding is order-dependant, i.e.
+       BEFORE INSERT OR UPDATE and BEFORE UPDATE OR INSERT will be coded differently,
+       although they have the same semantics and will be executed exactly the same way.
+    3. In multiple-action triggers both OLD and NEW contexts are available. If the
+       trigger invocation forbids one of them (e.g. OLD context for INSERT operation),
+       then all fields of that context will eveluate to NULL. If you assign to
+       unproper context, runtime exception will be thrown.
+    4. You may use new context variables INSERTING/UPDATEING/DELETING to check the
+       operation type at runtime.
+
+  See also:
+    README.context_variables