Doc: tweak textsearch.sgml for SEO purposes.

Google seems to like to return textsearch.html for queries about
GIN and GiST indexes, even though it's not a primary reference
for either.  It seems likely that that's because those keywords
appear in the page title.  Since "GIN and GiST Index Types" is
not a very apposite title for this material anyway, rename the
section in hopes of stopping that.

Also provide explicit links to the GIN and GiST chapters, to help
anyone who finds their way to this page regardless.

Per gripe from Jan Piotrowski.  Back-patch to supported branches.
(Unfortunately Google is likely to continue returning the 9.1
version of this page, but improving that situation is a matter
for the www team.)

Discussion: https://postgr.es/m/164978902252.1276550.9330175733459697101@wrigleys.postgresql.org

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