pgsql: Add argument names to the regexp_XXX functions.

Add argument names to the regexp_XXX functions.

This change allows these functions to be called using named-argument
notation, which can be helpful for readability, particularly for
the ones with many arguments.

There was considerable debate about exactly which names to use,
but in the end we settled on the names already shown in our
documentation table 9.10.

The citext extension provides citext-aware versions of some of
these functions, so add argument names to those too.

In passing, fix table 9.10's syntax synopses for regexp_match,
which were slightly wrong about which combinations of arguments
are allowed.

Jian He, reviewed by Dian Fay and others

Discussion: https://postgr.es/m/CACJufxG3NFKKsh6x4fRLv8h3V-HvN4W5dA=zNKMxsNcDwOKang@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/580f8727ca93b7b9a2ce49746b9cdbcb0a2b4a7e

Modified Files
--------------
contrib/citext/Makefile | 1 +
contrib/citext/citext--1.6--1.7.sql | 45 +++++++++++++++++++++++++
contrib/citext/citext.control | 2 +-
contrib/citext/meson.build | 1 +
doc/src/sgml/func.sgml | 26 ++++++++------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 67 ++++++++++++++++++++++++++-----------
7 files changed, 112 insertions(+), 32 deletions(-)