add comment to clarify corners cut in prototype

src/client.js

@@ -768,6 +768,14 @@ MatrixClient.prototype.loadRoomMembersIfNeeded = async function(roomId) {
         return;
     }
     const joinedMembersPromise = this.joinedMembers(roomId);
+    // XXX: we should make sure that the members we get back represent the
+    // room state at a given point in time. The plan is to do this by
+    // passing the current next_batch sync token to the endpoint we use
+    // to fetch the members. For now, this is a prototype that uses
+    // the /joined_members api, which does not support this synchronization,
+    // so there is a race condition here between the current /sync call
+    // and the /joined_members call: if the have conflicting information, which one
+    // represents the most recent state?
     const membersPromise = joinedMembersPromise.then((profiles) => {
         return Object.entries(profiles.joined).map(([userId, profile]) => {
             return {