docs: Add missing settings.ini Environment group keys
Backends, Drivers and SyncToVblank are available but not documented in "Running Clutter".
I've just interpreted these from clutter-main.c.
Looks like the CLUTTER_VBLANK explanation in other parts of the same page are out of date, no longer GLX specific and now a bool?
e8562089 ("main: Add a sync-to-vblank global flag", 2011-11-04)