Apply suggestions from code review

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

proposals/3874-messages-endpoint-threads-filter.md

@@ -16,7 +16,7 @@ this would allow clients significant usability improvements with threads.
 This proposal recommends extending the existing [Event filters] are extended with new filters, named `not_rel_types` and
 `rel_types`.  If the `rel_types` filter is specified, messages which match any of the given relation types will be
 returned. If the `not_rel_types` filter is specified, only messages which match none of the given relation types will be
-returned.
+returned, this includes events without a relation.
 
 If a relation type is present in both of these fields, `not_rel_types` takes precedence and messages with this type will
 not be returned.
@@ -36,15 +36,14 @@ formatted for legibility:
 }
 ```
 
-Note that the newly added filtering parameters return events based on information in related events. Consider the
-following events in a room:
+The newly added filtering parameters return events based on information in the `m.relates_to` field. Consider the following events in a room:
 
 * `A`: a `m.room.message` event sent by `alice`
 * `B`: a `m.room.message` event sent by `bob`
 * `C`: a `m.room.message` event sent by `charlie` which relates to `A` with type `m.thread`
 
 Using a filter of `"not_rel_types": ["m.thread"]` would return only events `A` and `B` as they do not have a relation of
-`m.thread` in them. Thread roots are returned in the same way as messages which are not part of threads at all.
+`m.thread` in them.
 
 ## Potential issues