doc: improve doc Http2Session:Timeout

Add line in doc/http2.md for 'timeout' event which tell readers
that 'timeout' event doesn't except any arguments.

Refs: https://github.com/nodejs/help/issues/877
PR-URL: https://github.com/nodejs/node/pull/30161


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>