doc: add notes to http.get options

Extra notes that options doesn't include the prototype when copied

Fixes: https://github.com/nodejs/node/issues/12092
PR-URL: https://github.com/nodejs/node/pull/12124


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>