pgsql: Mentino that PAM doesn't work for Unix system authentication

Log Message:
-----------
Mentino that PAM doesn't work for Unix system authentication because of
the problems non-root reading /etc/shadow.

Modified Files:
--------------
pgsql/doc/src/sgml:
client-auth.sgml (r1.100 -> r1.101)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/client-auth.sgml?r1=1.100&r2=1.101)