Doc: fix minor issues in GiST support function documentation.

Doc: fix minor issues in GiST support function documentation.

gist.sgml and xindex.sgml hadn't been fully updated for the
addition of a sortsupport support function (commit 16fa9b2b3).
xindex.sgml also missed that the compress and decompress support
functions are optional, an apparently far older oversight.

In passing, fix gratuitous inconsistencies in wording and
capitalization.

Noted by E. Rogov.  Back-patch to v14; the residual issues
before that aren't significant enough to bother with.

Discussion: https://postgr.es/m/163335322905.12519.5711557029494638051@wrigleys.postgresql.org