[media] doc-rst: 16-bit BGGR is always 16 bits

The V4L2_PIX_FMT_SBGGR16 format is documented to contain samples of fewer
than 16 bits. However, we do have specific definitions for smaller sample
sizes. Therefore, this note is redundant from the API point of view.

Currently only two drivers, am437x and davinci, use the
V4L2_PIX_FMT_SBGGR16 pixelformat currently. The sampling precision is
understood to be 16 bits in all current cases.

Remove the note on sampling precision.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>