doc: indicate that Http2ServerResponse extends Stream

Also removes the redundant statement explained by "extends Stream".

PR-URL: https://github.com/nodejs/node/pull/29254


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>