[media] DocBook/media/v4l: il_* fields always 0 for progressive formats

Clarify that the il_vfrontporch, il_vsync and il_vbackporch fields must
always be 0 for progressive formats.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

Documentation/DocBook/media/v4l/vidioc-g-dv-timings.xml

@@ -156,19 +156,19 @@ bit 0 (V4L2_DV_VSYNC_POS_POL) is for vertical sync polarity and bit 1 (V4L2_DV_H
 	    <entry>__u32</entry>
 	    <entry><structfield>il_vfrontporch</structfield></entry>
 	    <entry>Vertical front porch in lines for the even field (aka field 2) of
-	    interlaced field formats.</entry>
+	    interlaced field formats. Must be 0 for progressive formats.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>il_vsync</structfield></entry>
 	    <entry>Vertical sync length in lines for the even field (aka field 2) of
-	    interlaced field formats.</entry>
+	    interlaced field formats. Must be 0 for progressive formats.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>il_vbackporch</structfield></entry>
 	    <entry>Vertical back porch in lines for the even field (aka field 2) of
-	    interlaced field formats.</entry>
+	    interlaced field formats. Must be 0 for progressive formats.</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>