Add more context for why threaded vs unthreaded read receipts (#3378)

* Add more context for why threaded vs unthreaded read receipts

See #3339 (comment)

* Language updates from Andy

See #3378 (comment)

* Fix lints

src/receipt-accumulator.ts

@@ -118,6 +118,26 @@ export class ReceiptAccumulator {
  eventId,
  };
 
+ // In a world that supports threads, read receipts normally have
+ // a `thread_id` which is either the thread they belong in or
+ // `MAIN_ROOM_TIMELINE`, so we normally use `setThreaded(...)`
+ // here. The `MAIN_ROOM_TIMELINE` is just treated as another
+ // thread.
+ //
+ // We still encounter read receipts that are "unthreaded"
+ // (missing the `thread_id` property). These come from clients
+ // that don't support threads, and from threaded clients that
+ // are doing a "Mark room as read" operation. Unthreaded
+ // receipts mark everything "before" them as read, in all
+ // threads, where "before" means in Sync Order i.e. the order
+ // the events were received from the homeserver in a sync.
+ // [Note: we have some bugs where we use timestamp order instead
+ // of Sync Order, because we don't correctly remember the Sync
+ // Order. See #3325.]
+ //
+ // Calling the wrong method will cause incorrect behavior like
+ // messages re-appearing as "new" when you already read them
+ // previously.
  if (!data.thread_id) {
  this.setUnthreaded(userId, receipt);
  } else {