drm: document DRM_IOCTL_GEM_CLOSE

This is a bit tricky, because of the ref'counting considerations.
See also [1] for more discussion about this topic. Since this is
kernel docs, I've decided to elaborate a bit less on the user-space
details.

[1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/110



Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230216130934.156541-2-contact@emersion.fr