refactor func-matching.sgml, make regexp* function more readable

hi.

https://www.postgresql.org/docs/current/functions-matching.html#FUNCTIONS-POSIX-REGEXP

section3 way too big.
The first big part is all these (regexp*) functions, the second part
is (9.7.3.1. Regular Expression Details )

Please check the attached refactoring. output sample picture also attached.

All these functions (regexp_count, regexp_instr, regexp_like, regexp_match,
regexp_matches, regexp_replace, regexp_split_to_array, regexp_split_to_table,
regexp_substr) are explained/described one after another, which makes the
section (9.7.3 first part) feels a bit condensed.

So I’ve split them up and enclosed each in its own <sect4> section.
I used <synopsis> to mark up each function’s syntax, followed by a detailed
description. Since each function’s explanation is quite long, using <sect4> to
separate them seems more reasonable.

we could use <table> to group them, like
https://www.postgresql.org/docs/current/functions-json.html#SQLJSON-QUERY-FUNCTIONS
but we have many functions here, using <sect4> would be better than <table>.