element: Update gst_element_set_state() description
Since GST_MESSAGE_ASYNC_DONE might be used the wrong way following the current description and the usage of GST_MESSAGE_ASYNC_DONE is discouraged update the documentation.
Since GST_MESSAGE_ASYNC_DONE might be used the wrong way following the current description and the usage of GST_MESSAGE_ASYNC_DONE is discouraged update the documentation.