Misc clarifications to threading MSCs (#3899)

* Thread bundles exclude ignored users

Per implementation: matrix-org/synapse#12235

* Clarify that the threads list API returns a 403 too

Per implementation: matrix-org/synapse#13394

* Reference a token that actually exists

proposals/3440-threading-via-relations.md

@@ -51,12 +51,14 @@ would include additional information in the `unsigned` field:
 }
 ```
 
-* `latest_event`: The most recent event which relates to this event, with
-  `rel_type` of `m.thread`.
+* `latest_event`: The most recent event (topologically) which relates to this event, with
+  `rel_type` of `m.thread`. Events sent by [ignored users](https://spec.matrix.org/v1.3/client-server-api/#ignoring-users)
+  are not considered for the latest event.
 
   The latest event should be serialised in the same form as the event itself;
   this includes adding any bundled aggregations for the event (and applying edits).[^1]
-* `count`: An integer counting the number of `m.thread` events
+* `count`: An integer counting the number of `m.thread` events, excluding events sent
+  by [ignored users](https://spec.matrix.org/v1.3/client-server-api/#ignoring-users).
 * `current_user_participated`: A boolean flag, which is set to `true` if the
   current logged in user has participated in the thread. The user has participated if:
   * They created the current event.

proposals/3715-relations-parity-messages.md

@@ -75,7 +75,7 @@ previous calls (as well as `next_batch` values, as is currently specified). The
 [definition of the `from` parameter](https://spec.matrix.org/unstable/client-server-api/#get_matrixclientv1roomsroomidrelationseventid)
 is updated:
 
-> Can be a `from_batch` token **or `next_batch`** token from a previous call, or a
+> Can be a `next_batch` token **or `prev_batch`** token from a previous call, or a
 > returned `start` token from `/messages`, or a `next_batch` token from `/sync`.
 
 (Bold indicates new text.)

proposals/3856-threads-list-api.md

@@ -113,6 +113,10 @@ slightly. This has two situations:
    non-ignored user; with the caveat that the ordering of the threads is not
    re-arranged due to this replacement.
 
+Like [`/messages`](https://spec.matrix.org/v1.3/client-server-api/#get_matrixclientv3roomsroomidmessages),
+this returns a 403 `M_FORBIDDEN` error response if the caller isn't able to see the
+room's history.
+
 #### Example request:
 
 ```