doc: document URLSearchParams constructor

PR-URL: https://github.com/nodejs/node/pull/11060
Ref: https://github.com/whatwg/url/pull/175


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>