Clarify that threads can be created based on replies (#1687)

Signed-off-by: Andy Balaam <andy.balaam@matrix.org>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

changelogs/client_server/newsfragments/1687.clarification

@@ -0,0 +1 @@
+Clarify that threads can be created based on replies.

content/client-server-api/modules/threading.md

@@ -14,9 +14,11 @@ history behind a disclosure.
 Threads are established using a `rel_type` of `m.thread` and reference the
 *thread root* (the main timeline event to which the thread events refer). It is not possible to create a thread from an event which itself
 is the child of an event relationship (i.e., one with an `m.relates_to`
-property). It is therefore also not possible to nest threads. All events in a
-thread reference the thread root instead of the most recent message, unlike rich
-reply chains.
+property with a `rel_type` property - see [Relationship types](#relationship-types)).
+It is therefore also not possible to nest threads. 
+
+Unlike rich reply chains, all events in a thread reference the thread root
+instead of the most recent message.
 
 As a worked example, the following represents a thread and how it would be formed: