pgsql: Doc: minor improvements for section 11.2 "Index Types".

Doc: minor improvements for section 11.2 "Index Types".

Break the per-index-type discussions into <sect2>'s so as to make
them more visually separate and easier to find.  Improve the markup,
and make a couple of small wording adjustments.

This also fixes one stray reference to the now-deprecated point
operators <^ and >^.

Dagfinn Ilmari Mannsåker, reviewed by David Johnston and Jürgen Purtz

Discussion: https://postgr.es/m/877dukhvzg.fsf@wibble.ilmari.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/85b4ba73423b480902206ca04330c1cbea371c3c

Modified Files
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doc/src/sgml/indices.sgml | 126 +++++++++++++++++++++++++---------------------
1 file changed, 70 insertions(+), 56 deletions(-)