doc: document the ready event for Http2Stream

Fixes: #22804

PR-URL: #34221
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/http2.md

@@ -984,6 +984,15 @@ argument identifying the frame type, and an integer argument identifying the
 error code. The `Http2Stream` instance will be destroyed immediately after the
 `'frameError'` event is emitted.
 
+#### Event: `'ready'`
+<!-- YAML
+added: v8.4.0
+-->
+
+The `'ready'` event is emitted when the `Http2Stream` has been opened, has
+been assigned an `id`, and can be used. The listener does not expect any
+arguments.
+
 #### Event: `'timeout'`
 <!-- YAML
 added: v8.4.0