drm: document DRM_IOCTL_PRIME_HANDLE_TO_FD and PRIME_FD_TO_HANDLE

v2: mention caps, note that the IOCTLs might fail, document that
user-space needs a data structure to keep track of the
handles (Daniel V.)

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/20230217162151.59996-2-contact@emersion.fr