media: Documentation: media: Describe V4L2_CID_UNIT_CELL_SIZE

New control to pass to userspace the width/height of a pixel. Which is
needed for calibration and lens selection.
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/v4l/ext-ctrls-image-source.rst

@@ -55,3 +55,13 @@ Image Source Control IDs
 
 ``V4L2_CID_TEST_PATTERN_GREENB (integer)``
     Test pattern green (next to blue) colour component.
+
+``V4L2_CID_UNIT_CELL_SIZE (struct)``
+    This control returns the unit cell size in nanometers. The struct
+    :c:type:`v4l2_area` provides the width and the height in separate
+    fields to take into consideration asymmetric pixels.
+    This control does not take into consideration any possible hardware
+    binning.
+    The unit cell consists of the whole area of the pixel, sensitive and
+    non-sensitive.
+    This control is required for automatic calibration of sensors/cameras.