media: vivid.rst: document HDMI Video Guard Band control

Document this new vivid test control.
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab@kernel.org>

Documentation/admin-guide/media/vivid.rst

@@ -714,6 +714,20 @@ The Test Pattern Controls are all specific to video capture.
 
 	does the same for the EAV (End of Active Video) code.
 
+- Insert Video Guard Band
+
+	adds 4 columns of pixels with the HDMI Video Guard Band code at the
+	left hand side of the image. This only works with 3 or 4 byte RGB pixel
+	formats. The RGB pixel value 0xab/0x55/0xab turns out to be equivalent
+	to the HDMI Video Guard Band code that precedes each active video line
+	(see section 5.2.2.1 in the HDMI 1.3 Specification). To test if a video
+	receiver has correct HDMI Video Guard Band processing, enable this
+	control and then move the image to the left hand side of the screen.
+	That will result in video lines that start with multiple pixels that
+	have the same value as the Video Guard Band that precedes them.
+	Receivers that will just keep skipping Video Guard Band values will
+	now fail and either loose sync or these video lines will shift.
+
 
 Capture Feature Selection Controls
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^