[media] V4L2 Spec: clarify usage of V4L2_FBUF_FLAG_PRIMARY

As per discussion during the 2011 V4L-DVB workshop.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/vidioc-g-fbuf.xml

@@ -295,7 +295,8 @@ set this field to zero.</entry>
 	    <entry>The device is capable of non-destructive overlays.
 When the driver clears this flag, only destructive overlays are
 supported. There are no drivers yet which support both destructive and
-non-destructive overlays.</entry>
+non-destructive overlays. Video Output Overlays are in practice always
+non-destructive.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_FBUF_CAP_CHROMAKEY</constant></entry>
@@ -356,7 +357,9 @@ with the chroma-key colors are replaced by video pixels, which is exactly opposi
 	    <entry><constant>V4L2_FBUF_FLAG_PRIMARY</constant></entry>
 	    <entry>0x0001</entry>
 	    <entry>The framebuffer is the primary graphics surface.
-In other words, the overlay is destructive. [?]</entry>
+In other words, the overlay is destructive. This flag is typically set by any
+driver that doesn't have the <constant>V4L2_FBUF_CAP_EXTERNOVERLAY</constant>
+capability and it is cleared otherwise.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_FBUF_FLAG_OVERLAY</constant></entry>
@@ -366,9 +369,8 @@ size as the capture. [?]</entry>
 	  </row>
 	  <row>
 	    <entry spanname="hspan">The purpose of
-<constant>V4L2_FBUF_FLAG_PRIMARY</constant> and
 <constant>V4L2_FBUF_FLAG_OVERLAY</constant> was never quite clear.
-Most drivers seem to ignore these flags. For compatibility with the
+Most drivers seem to ignore this flag. For compatibility with the
 <wordasword>bttv</wordasword> driver applications should set the
 <constant>V4L2_FBUF_FLAG_OVERLAY</constant> flag.</entry>
 	  </row>