media: docs: Document metadata format in struct v4l2_format

The format fields in struct v4l2_format were otherwise documented but the
meta field was missing. Document it.

Reported-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>