[media] DocBook: Mark CROPCAP as optional instead of as compulsory

While the documentation says that VIDIOC_CROPCAP is compulsory for
all video capture and output devices, in practice VIDIOC_CROPCAP is
only implemented for devices that can do cropping and/or scaling.
Update the documentation to no longer require VIDIOC_CROPCAP if the
driver does not support cropping or scaling or non-square pixels.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>