media: uapi: pixfmt-luma: Document MIPI CSI-2 packing

The Y10P, Y12P and Y14P format variants are packed according to
the RAW10, RAW12 and RAW14 formats as defined by the MIPI CSI-2
specification. Document it.
Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

Documentation/userspace-api/media/v4l/pixfmt-yuv-luma.rst

@@ -209,3 +209,7 @@ are often referred to as greyscale formats.
     For Y012 and Y12 formats, Y012 places its data in the 12 high bits, with
     padding zeros in the 4 low bits, in contrast to the Y12 format, which has
     its padding located in the most significant bits of the 16 bit word.
+
+    The 'P' variations of the Y10, Y12 and Y14 formats are packed according to
+    the RAW10, RAW12 and RAW14 packing scheme as defined by the MIPI CSI-2
+    specification.