gerror: Mention that g_clear_error can be used with an "empty" GError

It is fine to do:
GError *error = NULL;
g_clear_error (&error);

https://bugzilla.gnome.org/show_bug.cgi?id=762937

glib/gerror.c

@@ -663,7 +663,7 @@ g_propagate_error (GError **dest,
  * g_clear_error:
  * @err: a #GError return location
  *
- * If @err is %NULL, does nothing. If @err is non-%NULL,
+ * If @err or *@err is %NULL, does nothing. Otherwise,
  * calls g_error_free() on *@err and sets *@err to %NULL.
  */
 void