doc: doc and test URLSearchParams discrepancy

The WHATWG URL spec is not going to change this behavior so
let's document it

Signed-off-by: James M Snell <jasnell@gmail.com>

Fixes: https://github.com/nodejs/node/issues/33037

PR-URL: https://github.com/nodejs/node/pull/33236


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>