doc: add documentation for the 'timeout' event

Adds the `'timeout'` event to the `http.ClientRequest` documentation.

PR-URL: https://github.com/nodejs/node/pull/15443
Fixes: https://github.com/nodejs/node/issues/14856


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>