add hint about system configuration file

htop.1.in

@@ -350,7 +350,9 @@ Currently unsupported (always displays '-').
 .LP 
 By default htop reads its configuration from the XDG-compliant path
 ~/.config/htop/htoprc -- the configuration file is overwritten by htop's
-in-program Setup configuration, so it should not be hand-edited.
+in-program Setup configuration, so it should not be hand-edited. If no
+user configuration exists htop tries to read system-wide configuration
+from @sysconfdir@/htoprc and falls back to hard coded default.
 .LP
 You may override the location of the configuration file using the $HTOPRC
 environment variable (so you can have multiple configurations for different