Doc: update documentation about EXCLUDE constraint elements.

Doc: update documentation about EXCLUDE constraint elements.

What the documentation calls an exclude_element is an index_elem
according to gram.y, and it allows all the same options that
a CREATE INDEX column specification does.  The COLLATE patch
neglected to update the CREATE/ALTER TABLE docs about that,
and later the opclass-parameters patch made the same oversight.
Add those options to the syntax synopses, and polish the
associated text a bit.

Back-patch to v13 where opclass parameters came in.  We could
update v12 with just the COLLATE omission, but it doesn't quite
seem worth the trouble at this point.

Shihao Zhong, reviewed by Daniel Vérité, Shubham Khanna and myself

Discussion: https://postgr.es/m/CAGRkXqShbVyB8E3gapfdtuwiWTiK=Q67Qb9qwxu=+-w0w46EBA@mail.gmail.com