doc: Document pg_trgm being case-insensitive by default

pg_trgm is by default operating case-insensitively but the docs didn't
mention that at all.

Author: Erik Rijkers <er@xs4all.nl>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reported-by: marcmaiwald@googlemail.com
Discussion: https://postgr.es/m/166064504415.652.12724576876807446945@wrigleys.postgresql.org

Author: danielgustafsson

doc/src/sgml/pgtrgm.sgml

@@ -416,6 +416,8 @@
    the above-described similarity operators, and additionally support
    trigram-based index searches for <literal>LIKE</literal>, <literal>ILIKE</literal>,
    <literal>~</literal>, <literal>~*</literal> and <literal>=</literal> queries.
+   The similarity comparisons are case-insensitive in a default build of
+   <filename>pg_trgm</filename>.
    Inequality operators are not supported.
    Note that those indexes may not be as efficient as regular B-tree indexes
    for equality operator.