Docs: assorted minor cleanups.

Standardize on "user_name" for a field name in related examples in
ddl.sgml; before we had variously "user_name", "username", and "user".
The last is flat wrong because it conflicts with a reserved word.

Be consistent about entry capitalization in a table in func.sgml.

Fix a typo in pgtrgm.sgml.

Back-patch to 9.6 and 9.5 as relevant.

Alexander Law