Clarify which transaction ID

event-schemas/schema/core-event-schema/unsigned_prop.yaml

@@ -27,6 +27,8 @@ properties:
     title: Event
     type: object
   transaction_id:
-    description: The client-supplied transaction ID, if the client being given
+    description: |
-      the event is the same one which sent it.
+      The client-supplied transaction ID, for example, provided via
+      ``PUT /_matrix/client/r0/rooms/{roomId}/send/{eventType}/{txnId}``,
+      if the client being given the event is the same one which sent it.
     type: string