document that not all methods can be combined

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>

doc/sample.config

@@ -1,7 +1,9 @@
 # User authentication method. Could be set multiple times and in 
 # that case all should succeed. To enable multiple methods use
 # multiple auth directives. Available options: certificate, 
-# plain, pam, radius, gssapi.
+# plain, pam, radius, gssapi. Note that authentication methods
+# utilizing passwords cannot be combined (e.g., the plain, pam
+# or radius methods).
 #
 # Note that authentication methods cannot be changed with reload.