drm/probe-helper: Document drm_helper_hpd_irq_event() return value

The documentation of the drm_helper_hpd_irq_event() function didn't
document the value that function was returning. Add that part as well.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914101724.266570-1-maxime@cerno.tech

drivers/gpu/drm/drm_probe_helper.c

@@ -817,6 +817,9 @@ EXPORT_SYMBOL(drm_kms_helper_poll_fini);
  *
  * Note that a connector can be both polled and probed from the hotplug handler,
  * in case the hotplug interrupt is known to be unreliable.
+ *
+ * Returns:
+ * A boolean indicating whether the connector status changed or not
  */
 bool drm_helper_hpd_irq_event(struct drm_device *dev)
 {