Re-add doc on scroll state map structure

src/stores/RoomScrollStateStore.js

@@ -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 = {};
     }