add changelog

Author: TomStrepsil

doc/api/webstreams.md

@@ -12,6 +12,11 @@ changes:
   - version: v18.0.0
     pr-url: https://github.com/nodejs/node/pull/42225
     description: Use of this API no longer emit a runtime warning.
+  - version:
+    - v21.5.0
+    - v20.14.0
+    pr-url: https://github.com/nodejs/node/pull/50126
+    description: Supports the `cancel` transformer callback.
 -->
 
 > Stability: 2 - Stable