pgsql: Doc: update section 9.11 for new function table layout.

Doc: update section 9.11 for new function table layout.

This also makes an attempt to flesh out the docs for some of the more
severely underdocumented geometric operators and functions.

This effort exposed that the point <^ point (point_below) and
point >^ point (point_above) operators are misnamed; they should be
<<| and |>>, because they act like the other operators named that
way and not like the other operators named <^ and >^. But I just
documented them that way; fixing it is matter for another patch.

The haphazard datatype coverage of many of the operators is also
now depressingly obvious.

Discussion: https://postgr.es/m/158110996889.1089.4224139874633222837@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/791090bd775b6a2b488ae2078c8479fcd3324a2c

Modified Files
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doc/src/sgml/func.sgml | 1592 +++++++++++++++++++++++++++++++++---------------
1 file changed, 1094 insertions(+), 498 deletions(-)