[media] DocBook: improve STREAMON/OFF documentation

Specify that STREAMON/OFF should return 0 if the stream is already
started/stopped.
The spec never specified what the correct behavior is. This ambiguity
was resolved during the 2012 Media Workshop.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/vidioc-streamon.xml

@@ -74,7 +74,12 @@ not transmitted yet. I/O returns to the same state as after calling
 stream type. This is the same as &v4l2-requestbuffers;
 <structfield>type</structfield>.</para>
 
-    <para>Note applications can be preempted for unknown periods right
+    <para>If <constant>VIDIOC_STREAMON</constant> is called when streaming
+is already in progress, or if <constant>VIDIOC_STREAMOFF</constant> is called
+when streaming is already stopped, then the ioctl does nothing and 0 is
+returned.</para>
+
+    <para>Note that applications can be preempted for unknown periods right
 before or after the <constant>VIDIOC_STREAMON</constant> or
 <constant>VIDIOC_STREAMOFF</constant> calls, there is no notion of
 starting or stopping "now". Buffer timestamps can be used to