doc: mention yum opts in qubes-dom0-update manual page

doc/tools/qubes-dom0-update.rst

@@ -10,7 +10,7 @@ qubes-dom0-update - update software in dom0
 
 SYNOPSIS
 ========
-| qubes-dom0-update [--clean] [--check-only] [--gui] [<pkg list>]
+| qubes-dom0-update [--clean] [--check-only] [--gui] [<yum opts>] [<pkg list>]
 
 OPTIONS
 =======
@@ -24,6 +24,10 @@ OPTIONS
 <pkg list>
     Download (and install if run by root) new packages in dom0 instead of updating
 
+Besides above options, when no --gui or --check-only given, all other options
+are passed to yum call. So for example --enablerepo/--disablerepo options works
+as well.
+
 AUTHORS
 =======
 | Joanna Rutkowska <joanna at invisiblethingslab dot com>