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

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