Add ideas of pending replies

doc/impl-thoughts/PENDING_REPLIES.md

@@ -0,0 +1,25 @@
+# Replying to pending messages
+The matrix spec requires clients capable of rich replies (that would be us once replies work) to include fallback (textual in `body` and structured in `formatted_body`) that can be rendered
+by clients that do not natively support rich replies (that would be us at the time of writing). The schema for the fallback is as follows:
+
+```
+<mx-reply>
+  <blockquote>
+    <a href="https://matrix.to/#/!somewhere:example.org/$event:example.org">In reply to</a>
+    <a href="https://matrix.to/#/@alice:example.org">@alice:example.org</a>
+    <br />
+    <!-- This is where the related event's HTML would be. -->
+  </blockquote>
+</mx-reply>
+```
+
+There's a single complication here for pending events: we have `$event:example.org` in the schema (the `In reply to` link), and it must
+be present _within the content_, inside `formatted_body`. The issue is that, if we are queuing a reply to a pending event,
+we don't know its remote ID. All we know is its transaction ID on our end. If we were to use that while formatting the message,
+we'd be sending messages that contain our internal transaction IDs instead of proper matrix event identifiers.
+
+To solve this, we'd need `SendQueue`, whenever it receives a remote echo, to update pending events that are replies with their
+`relatedEventId`. This already happens, and the `event_id` field in `m.relates_to` is updated. But we'd need to extend this
+to adjust the messages' `formatted_body` with the resolved remote ID, too.
+
+How do we safely do this, without accidentally substituting event IDs into places in the body where they were not intended?