[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>

Documentation/media/uapi/v4l/dev-codec.rst

@@ -26,7 +26,7 @@ parameters
    The MPEG controls actually support many more codecs than
    just MPEG. See :ref:`mpeg-controls`.
 
-Memory-to-memory devices can often be used as a shared resource: you can
+Memory-to-memory devices function as a shared resource: you can
 open the video node multiple times, each application setting up their
 own codec properties that are local to the file handle, and each can use
 it independently from the others. The driver will arbitrate access to

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 */