message: Add API description comment.
diff: @@ -14,6 +14,61 @@ See the License for the specific language governing permissions and
  limitations under the License.
  */
  +/*
  +Listens for incoming postMessage requests from embedded widgets. The following API is exposed:
  +{
  + widgetData: {
  + action: "content_loaded"
  + // additional request fields
  + },
  + widgetId: $WIDGET_ID
  +}
  +
  +The complete request object is returned to the caller with an additional "response" key like so:
  +{
  + widgetData: {
  + action: "content_loaded"
  + // additional request fields
  + },
  + widgetId: $WIDGET_ID
  + response: { ... }
  +}
  +
  +The "action" determines the format of the request and response. All actions can return an error response.
  +An error response is a "response" object which consists of a sole "error" key to indicate an error.
  +They look like:
  +{
  + error: {
  + message: "Unable to invite user into room.",
  + _error: <Original Error Object>
  + }
  +}
  +The "message" key should be a human-friendly string.
  +
  +ACTIONS
  +=======
  +All actions can return an error response instead of the response outlined below.
  +
  +content_loaded
  +--------------
  +Indicates that widget contet has fully loaded
  +
  +Request:
  + - widgetId is the unique ID of the widget instance in riot / matrix state.
  + - No additional fields.
  +Response:
  +{
  + success: true
  +}
  +Example:
  +{
  + widgetData: {
  + action: "content_loaded"
  + },
  + widgetId: $WIDGET_ID
  +}
  +*/
  +
  import dis from './dispatcher';
  let listenerCount = 0;
  