[media] v4l: Document that m2m devices have a file handle specific context

Memory-to-memory V4L2 devices all have file handle specific context.
Say this in the API documentation so that the user space may rely on it
being the case.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>