doc: add missing extends Http2Session

Adds missing extends Http2Session for ClientHttp2Session
and ServerHttp2Session.

PR-URL: #29252
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: trivikr

doc/api/http2.md

@@ -620,6 +620,8 @@ instance's underlying [`net.Socket`].
 added: v8.4.0
 -->
 
+* Extends: {Http2Session}
+
 #### serverhttp2session.altsvc(alt, originOrStream)
 <!-- YAML
 added: v9.4.0
@@ -742,6 +744,8 @@ server.on('stream', (stream) => {
 added: v8.4.0
 -->
 
+* Extends: {Http2Session}
+
 #### Event: 'altsvc'
 <!-- YAML
 added: v9.4.0