Adds PASSWORD_EXPIRES info to qmail-smtpd man page.

diff -ruN qmail-1.03-7.05/qmail-smtpd.8 qmail-1.03-7.06/qmail-smtpd.8
--- qmail-1.03-7.05/qmail-smtpd.8	2008-05-12 14:26:13.000000000 -0400
+++ qmail-1.03-7.06/qmail-smtpd.8	2008-01-22 14:14:26.000000000 -0500
@@ -519,6 +519,17 @@
 makes it possible to change its value using an AUTH_SET_MFCHECK
 environment variable.
 .TP 5
+.I PASSWORD_EXPIRES
+If this variable is set, it is expected to contain a unix time() value (the
+number of seconds since 1970-01-01 00:00:00 GMT.) If the current time is
+equal to or greater than this value, any AUTH command will fail with an
+error message saying that the password is expired.
+
+Logically, this variable should only be used on a per-user basis. The
+AUTH_CDB patch allows you to create environment variables which are
+created when a specific user sends a successful AUTH command. Otherwise,
+there is currently no other way to set this on a per-user basis.
+.TP 5
 .I QMAILQUEUE
 If this variable is set, it should point to a program which will be 
 executed instead of the