pgsql: Doc: split functions-posix-regexp section into multiple subsecti

Doc: split functions-posix-regexp section into multiple subsections.

Create a <sect4> section for each function that the previous text
described in one long series of paragraphs. Also split the functions'
previously in-line syntax summaries into <synopsis> clauses, which is
more readable and allows us to sneak in an explicit mention of the
result data type.

This change gives us an opportunity to make cross-reference links
more specific, too, so do that.

Author: jian he <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CACJufxFuk9P=P4=BZ=qCkgvo6im8aL8NnCkjxx2S2MQDWNdouw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/00c025a001170979e99706ce746f75fcc615761d

Modified Files
--------------
doc/src/sgml/func/func-json.sgml | 2 +-
doc/src/sgml/func/func-matching.sgml | 255 +++++++++++++++++++++--------------
doc/src/sgml/func/func-string.sgml | 22 +--
doc/src/sgml/ref/psql-ref.sgml | 2 +-
4 files changed, 168 insertions(+), 113 deletions(-)