doc/url: various improvements to WHATWG API

Also remove executable bit from doc/api/url.md's mode.

PR-URL: nodejs#11330
Fixes: 4757ddc "doc: add basic documentation for WHATWG URL API"
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>