media: Clarify how menus are hidden by SUBDRV_AUTOSELECT

Some users have been having a hard time finding the hidden
menus. A typically case are camera sensor drivers
(e.g IMX219, OV5645, etc), which are common on embedded
platforms and not really "ancillary" devices.

The problem with MEDIA_SUBDRV_AUTOSELECT seems to be related
to the fact that it uses the "visible" syntax to hide
the menus.

This is not obvious and it normally takes some time to
figure out.

To fix the problem, add a comment on each of hidden menus,
which should clarify what option is causing menus to be hidden.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>