Added the docs

doc/sql.extensions/README.partial_indices

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+---------------
+Partial indices
+---------------
+
+  Function:
+    Allow to index only a subset of table rows defined by the search condition specified during index creation.
+
+  Author:
+    Dmitry Yemanov <dimitr@firebirdsql.org>
+
+  Syntax rules:
+    CREATE [UNIQUE] [{ASC[ENDING] | DESC[ENDING]}] INDEX <index_name> ON <table_name>
+      { (<column_list>) | COMPUTED [BY] ( <value_expression> ) }
+      WHERE <search_condition>
+
+  Scope:
+    DSQL (DDL)
+
+  Example(s):
+    1. CREATE INDEX IT1_COL ON T1 (COL) WHERE COL < 100;
+       SELECT * FROM T1 WHERE COL < 100;
+       -- PLAN (T1 INDEX (IT1_COL))
+    2. CREATE INDEX IT1_COL2 ON T1 (COL) WHERE COL IS NOT NULL;
+       SELECT * FROM T1 WHERE COL > 100;
+       -- PLAN (T1 INDEX IT1_COL2)
+    3. CREATE INDEX IT1_COL3 ON T1 (COL) WHERE COL = 1 OR COL = 2;
+       SELECT * FROM T1 WHERE COL = 2;
+       -- PLAN (T1 INDEX IT1_COL3)
+
+  Note(s):
+    1. A partial index definition may include the UNIQUE specification. In this case,
+       every key in the index is required to be unique. This allows to enforce uniqueness
+       across some subset of table rows.
+    2. Partial index is usable only in the following cases:
+       a) WHERE condition includes exactly the same boolean expression as the one
+          defined for the index;
+       b) Search condition defined for the index contains ORed boolean expressions
+          and one of them is explicitly included in the WHERE condition;
+       c) Search condition defined for the index specifies IS NOT NULL and the
+          WHERE condition includes an expression on the same field that is known to
+          ignore NULLs.
+