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
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master

Details
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https://git.postgresql.org/pg/commitdiff/54fbd155cc6fdbf875185035b3d9823f739b617d

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