drm: document DRM_MODE_PAGE_FLIP_EVENT interactions with atomic

It's not obvious off-hand which CRTCs will get a page-flip event
when using this flag in an atomic commit, because it's all
implicitly implied based on the contents of the atomic commit.
Document requirements for using this flag and how to request an
event for a CRTC.

Note, because prepare_signaling() runs right after
drm_atomic_set_property() calls, page-flip events are not delivered
for CRTCs pulled in later by DRM core (e.g. on modeset by
drm_atomic_helper_check_modeset()) or the driver (e.g. other CRTCs
sharing a DP-MST connector).

v2: fix cut off sentence in commit message (Pekka)

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Simona Vetter <simona@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: David Turner <david.turner@raspberrypi.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Link: https://lore.kernel.org/r/20250501112945.6448-1-contact@emersion.fr

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