pgsql: Doc: add a short summary of available authentication methods.

Doc: add a short summary of available authentication methods.

The "auth-methods" <sect1> used to include descriptions of all our
authentication methods.  Commit 56811e573 promoted its child <sect2>'s
to <sect1>'s, which has advantages but also created some issues:
* The auth-methods page itself is essentially empty/useless.
* Links that pointed to "auth-methods" as a placeholder for all
auth methods were rendered a bit nonsensical.
* DocBook no longer provides a subsection table-of-contents here,
which formerly was a useful if terse summary of available auth methods.

To improve matters, add a handwritten list of all the auth methods.

Per gripe from Dave Cramer.  Back-patch to v11 where the previous
commit came in.

Discussion: https://postgr.es/m/CADK3HH+xQLhcPgg=kWqfogtXGGZr-JdSo=x=WQC0PkAVyxUWyQ@mail.gmail.com

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e5a37d958decfbc9dc650e66775db5896b7486ff

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 95 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 94 insertions(+), 1 deletion(-)