doc: url.format - true slash postfix behaviour

Change url.format's references to slash postfixing to reflect
true behaviour (it only automatically postfixes slashes to the
slashedProtocols when host is present).

Fixes: #3361

PR-URL: https://github.com/nodejs/node/pull/4119


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>