[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>

include/media/v4l2-mem2mem.h

@@ -90,6 +90,9 @@ struct v4l2_m2m_queue_ctx {
  *		%TRANS_QUEUED, %TRANS_RUNNING and %TRANS_ABORT.
  * @finished: Wait queue used to signalize when a job queue finished.
  * @priv: Instance private data
+ *
+ * The memory to memory context is specific to a file handle, NOT to e.g.
+ * a device.
  */
 struct v4l2_m2m_ctx {
 	/* optional cap/out vb2 queues lock */