Documentation: vkms: clarify devres managed reference cleanup

added documentation to drm_dev_unregister clarifying that devres managed
devices allocated with devm_drm_dev_alloc do not require calls to
drm_dev_put.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Brandon Pollack <brpol@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230425080240.3582324-1-brpol@chromium.org

drivers/gpu/drm/drm_drv.c

@@ -969,7 +969,9 @@ EXPORT_SYMBOL(drm_dev_register);
  *
  * Unregister the DRM device from the system. This does the reverse of
  * drm_dev_register() but does not deallocate the device. The caller must call
- * drm_dev_put() to drop their final reference.
+ * drm_dev_put() to drop their final reference, unless it is managed with devres
+ * (as devices allocated with devm_drm_dev_alloc() are), in which case there is
+ * already an unwind action registered.
  *
  * A special form of unregistering for hotpluggable devices is drm_dev_unplug(),
  * which can be called while there are still open users of @dev.