Documentation: media: add missing V4L2_BUF_CAP_ flags

The documentation for V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS and
V4L2_BUF_CAP_SUPPORTS_REMOVE_BUFS was missing. Add this.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>