pgsql: Mention that GSSAPI is preferred over Kerberos authentication

Log Message:
-----------
Mention that GSSAPI is preferred over Kerberos authentication because
GSSAPI is industry-standard.

Modified Files:
--------------
pgsql/doc/src/sgml:
release.sgml (r1.536 -> r1.537)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.536&r2=1.537)