[krb5-auth-dialog] Improve description of --auto

[Guido Günther <guidog src gnome org>]

commit b2da1f9ef7ed6ade542dc105ac1185436faf3db7
Author: Guido Günther <agx sigxcpu org>
Date:   Fri Jul 7 11:22:33 2017 +0200

    Improve description of --auto
    
    Thanks: Daniel Richard G.

 src/krb5-auth-dialog.1.in |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/src/krb5-auth-dialog.1.in b/src/krb5-auth-dialog.1.in
index 2a9867e..7630fd5 100644
--- a/src/krb5-auth-dialog.1.in
+++ b/src/krb5-auth-dialog.1.in
@@ -17,8 +17,11 @@ credentials, prompting the user for whatever information is necessary.
 
 .TP
 \fB--auto\fR
-If this option is specified, \fBkrb5-auth-dialog\fR will exit if it finds that the
-user has no Kerberos credentials.
+If this option is specified, \fkrb5-auth-dialog\f will start only if it
+finds that the user has Kerberos credentials. This may be used to
+ensure that only users who actually use Kerberos see the program.
+Note that in some settings, users may authenticate to Kerberos at a
+later time even if they do not have Kerberos credentials initially.
 
 .SH "SEE ALSO"
 .BR krb5.conf (5)