message: Re-add doc on scroll state map structure
diff: @@ -19,6 +19,19 @@ limitations under the License.
  */
  class RoomScrollStateStore {
  constructor() {
  + // A map from room id to scroll state.
  + //
  + // If there is no special scroll state (ie, we are following the live
  + // timeline), the scroll state is null. Otherwise, it is an object with
  + // the following properties:
  + //
  + // focussedEvent: the ID of the 'focussed' event. Typically this is
  + // the last event fully visible in the viewport, though if we
  + // have done an explicit scroll to an explicit event, it will be
  + // that event.
  + //
  + // pixelOffset: the number of pixels the window is scrolled down
  + // from the focussedEvent.
  this._scrollStateMap = {};
  }
  