doc, test: add note to response.getHeaders

* also correct language for the same note for querystring.parse
* add assertions for said note

PR-URL: https://github.com/nodejs/node/pull/12887
Fixes: https://github.com/nodejs/node/issues/12885
Refs: https://github.com/nodejs/node/pull/12883


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>