drm/connector: add ref to drm_connector_get in iter docs

Mention that connectors need to be referenced manually if they are
to be accessed after the iteration has progressed or ended.

Signed-off-by: Simon Ser <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/drm_connector.h

···
  * drm_connector_list_iter_begin(), drm_connector_list_iter_end() and
  * drm_connector_list_iter_next() respectively the convenience macro
  * drm_for_each_connector_iter().
+ *
+ * Note that the return value of drm_connector_list_iter_next() is only valid
+ * up to the next drm_connector_list_iter_next() or
+ * drm_connector_list_iter_end() call. If you want to use the connector later,
+ * then you need to grab your own reference first using drm_connector_get().
  */
 struct drm_connector_list_iter {
 /* private: */