doc: update description of 'clientError' event

Default behavior is to send a '400 Bad Request' response if the socket
is writable.

PR-URL: https://github.com/nodejs/node/pull/18885
Refs: https://github.com/nodejs/node/pull/15324


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>