Clarify clients behavior on wl_keyboard.leave

Clients must assume that every key, including modifiers, are lifted
after getting wl_keyboard.leave event, not doing so could lead to key repeat
keep going after the user changes focus, or stuck modifiers, when the user
interacts with client without keyboard focus.

Signed-off-by: Kirill Chibisov <contact@kchibisov.com>