Add info about ignored users.

proposals/3855-threads-list-api.md

@@ -31,7 +31,6 @@ threads that the user has participated in (as defined in
 A client could add the user's MXID to the filter, e.g. `"related_by_senders":["@alice:example.com"]`,
 but this misses threads where the user sent the root message and has not yet replied.
 
-
 ## Proposal
 
 ### Client-Server endpoint
@@ -98,6 +97,11 @@ Query Parameters:
   A token corresponding to the start of `chunk`. This will be the same as the
   value given in `from`.
 
+XXX Define how ignored users are handled, which has two cases:
+
+1. If the ignored user sent the root thread event.
+2. If the ignored user sent the latest thread event.
+
 #### Example request:
 
 ```