drm/atomic: Make the drm_atomic_state documentation less ambiguous

drm/atomic: Make the drm_atomic_state documentation less ambiguous

The current documentation of drm_atomic_state says that it's the "global
state object". This is confusing since, while it does contain all the
objects affected by an update and their respective states, if an object
isn't affected by this update it won't be part of it.

Thus, it's not truly a "global state", unlike object state structures
that do contain the entire state of a given object.

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://lore.kernel.org/r/20231214100917.277842-4-mripard@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>