pgsql: Doc: update section 9.12 for new function table layout.

pgsql: Doc: update section 9.12 for new function table layout.

Doc: update section 9.12 for new function table layout.

Also rearrange that page a bit for more consistency and less
duplication.

In passing, fix erroneous examples of the results of abbrev(cidr)
in datatype.sgml, and do a bit of copy-editing there.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/5b0aa112a8f74e93d28c2dc002cfdaea5c40eeda

Modified Files
--------------
doc/src/sgml/datatype.sgml |  32 +-
doc/src/sgml/func.sgml     | 888 +++++++++++++++++++++++++--------------------
2 files changed, 510 insertions(+), 410 deletions(-)