media: docs-rst: v4l: Document V4L2_BUF_TYPE_META_OUTPUT interface

Document the interface for metadata output, including
V4L2_BUF_TYPE_META_OUTPUT buffer type and V4L2_CAP_META_OUTPUT capability
bits.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Tested-by: Tian Shu Qiu <tian.shu.qiu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>