drm/fb: document dirty helper better

Apparently little known fact that there's no need to hand-roll your own
anymore. Cc'ing a bunch of driver people who might want to know this
too.

v2: s/none/known/ (Chris Wilson)

Cc: Rob Clark <[e-mail]>
Cc: Sebastian Reichel <[e-mail]>
Cc: Tomi Valkeinen <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: Gerd Hoffmann <[e-mail]>
Cc: David Lechner <[e-mail]>
Cc: Noralf Trønnes <[e-mail]>
Cc: Hans de Goede <[e-mail]>
Reviewed-by: Noralf Trønnes <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/drm_framebuffer.h

···
 	 * for more information as all the semantics and arguments have a one to
 	 * one mapping on this function.
 	 *
+	 * Atomic drivers should use drm_atomic_helper_dirtyfb() to implement
+	 * this hook.
+	 *
 	 * RETURNS:
 	 *
 	 * 0 on success or a negative error code on failure.