drm: document expectations for GETFB2 handles

There are two important details missing from the docs:

- If the memory object backing the FB already has a GEM handle,
  it's not re-used, a new one is generated.
- Aliased planes will return the same GEM handle.

v2: document how user-space can obtain DMA-BUF FDs without leaking
handles (Pekka)

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>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230217162151.59996-1-contact@emersion.fr