drm: document uAPI page-flip flags

Document flags accepted by the page-flip and atomic IOCTLs.

v2 (Pekka):
- Mention DRM_EVENT_FLIP_COMPLETE in DRM_MODE_PAGE_FLIP_EVENT docs.
- Expand DRM_MODE_ATOMIC_NONBLOCK and DRM_MODE_ATOMIC_ALLOW_MODESET
  description.
v3:
- Fix struct field ref syntax (Daniel)
- Clarify when artifacts are no longer displayed (Daniel)
- Add note about sinks deciding to show artifacts on their own (Pekka, Daniel)
v4:
- Fix typo (Pekka)

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/505107/