pgsql: Issue a proper error message when MD5 is attempted when

Log Message:
-----------
Issue a proper error message when MD5 is attempted when
db_user_namespace is enabled.

Also document this limitation.

Modified Files:
--------------
pgsql/doc/src/sgml:
client-auth.sgml (r1.112 -> r1.113)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/client-auth.sgml?r1=1.112&r2=1.113)
config.sgml (r1.195 -> r1.196)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.195&r2=1.196)
pgsql/src/backend/libpq:
auth.c (r1.173 -> r1.174)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c?r1=1.173&r2=1.174)
hba.c (r1.174 -> r1.175)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c?r1=1.174&r2=1.175)