doc: clarify http.Agent constructor options

PR-URL: https://github.com/nodejs/node/pull/26412
Fixes: https://github.com/nodejs/node/issues/26357


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>