pgsql: Clarify documentation about username mapping when authenticating

Log Message:
-----------
Clarify documentation about username mapping when authenticating with
GSSAPI or Kerberos.

Ian Turner

Modified Files:
--------------
pgsql/doc/src/sgml:
client-auth.sgml (r1.131 -> r1.132)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/client-auth.sgml?r1=1.131&r2=1.132)