Input: document inhibiting

Document inhibiting input devices and its relation to being
a wakeup source.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20200617101822.8558-1-andrzej.p@collabora.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>