[media] DocBook: clarify that sequence is also set for output devices

It was not entirely obvious that the sequence count should also
be set for output devices. Also made it more explicit that this
sequence counter counts frames, not fields.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: NSylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: NSakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/io.xml

@@ -613,8 +613,8 @@ field is independent of the <structfield>timestamp</structfield> and
 	    <entry>__u32</entry>
 	    <entry><structfield>sequence</structfield></entry>
 	    <entry></entry>
-	    <entry>Set by the driver, counting the frames in the
-sequence.</entry>
+	    <entry>Set by the driver, counting the frames (not fields!) in
+sequence. This field is set for both input and output devices.</entry>
 	  </row>
 	  <row>
 	    <entry spanname="hspan"><para>In <link