protocol: make it clear wl_surface.enter/leave are not for frame throttling

Some clients rely on wl_surface.enter/leave to start/stop their rendering
loop.

There are cases where this doesn't work:

- Some compositors don't send wl_surface.leave when a toplevel is hidden.
  For instance Sway doesn't send this event when a toplevel is on an inactive
  workspace (but doesn't send wl_surface.frame events).
- Some compositors might still want applications to continue to render even if
  away from outputs. For instance a compositor that allows to screen record
  individual toplevels might not send a wl_surface.enter event for hidden
  toplevels.

Signed-off-by: Simon Ser <contact@emersion.fr>