Doc client_min_messages patch vis. INFO message severity

Hi,

Attached is documentation patch: doc_client_min_messages_v1.patch

Document that INFO severity messages are always sent
to the client.  This also adds hyperlinks to the
table of severity levels where those levels are
referenced in the docs.

The patch was discussed on the #postgresql IRC channel
with RhodiumToad.

The patch is against master.  It passes the xmllint,
builds html, and generally appears to work.

The motivation for this patch is that the
client_min_messages documentation does not mention
the INFO level.

Regards,

Karl <kop@meme.com>
Free Software:  "You don't pay back, you pay forward."
                 -- Robert A. Heinlein