As per commit ad72fa46b clutter_x11_handle_event was renamed into
meta_x11_handle_event but the return type didn't change.
So, keep the doc-string to match the actual possible return values.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/764
The end goal is to have all clutter backend code in src/backends. Input
is the larger chunk of it, which is now part of our specific
MutterClutterBackendX11, this extends to device manager, input devices,
tools and keymap.
This was supposed to be nice and incremental, but there's no sane way
to cut this through. As a result of the refactor, a number of private
Clutter functions are now exported for external backends to be possible.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/672
