media: videobuf2: Add missing doc comment for waiting_in_dqbuf

While at it rearrange other comments to match the order of struct members.

Fixes: d65842f7

 ("media: vb2: add waiting_in_dqbuf flag")

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>