[media] DocBook: media: Document ALPHA_COMPONENT control usage on output devices

Extend the V4L2_CID_ALPHA_COMPONENT control for use on output devices,
to set the alpha component value when the output format doesn't have an
alpha channel.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

Documentation/DocBook/media/v4l/controls.xml

@@ -408,14 +408,17 @@ to work.</entry>
 	  <row id="v4l2-alpha-component">
 	    <entry><constant>V4L2_CID_ALPHA_COMPONENT</constant></entry>
 	    <entry>integer</entry>
-	    <entry> Sets the alpha color component on the capture device or on
-	    the capture buffer queue of a mem-to-mem device. When a mem-to-mem
-	    device produces frame format that includes an alpha component
+	    <entry>Sets the alpha color component. When a capture device (or
+	    capture queue of a mem-to-mem device) produces a frame format that
+	    includes an alpha component
 	    (e.g. <link linkend="rgb-formats">packed RGB image formats</link>)
-	    and the alpha value is not defined by the mem-to-mem input data
-	    this control lets you select the alpha component value of all
-	    pixels. It is applicable to any pixel format that contains an alpha
-	    component.
+	    and the alpha value is not defined by the device or the mem-to-mem
+	    input data this control lets you select the alpha component value of
+	    all pixels. When an output device (or output queue of a mem-to-mem
+	    device) consumes a frame format that doesn't include an alpha
+	    component and the device supports alpha channel processing this
+	    control lets you set the alpha component value of all pixels for
+	    further processing in the device.
 	    </entry>
 	  </row>
 	  <row>