drm/atomic: Document history of drm_atomic_state

drm/atomic: Document history of drm_atomic_state

After some discussions on the mailing-list for an earlier revision of
the series, it was suggested to document the evolution of
drm_atomic_state and its use by drivers to explain some of the confusion
one might still encounter when reading the framework code.

Suggested-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/dri-devel/Z4jtKHY4qN3RNZNG@phenom.ffwll.local/
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250213-bridge-connector-v3-1-e71598f49c8f@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>