doc: add `servername` parameter docs

Add `servername` parameter docs for `https.request()` method.

Follows #4389

PR-URL: https://github.com/nodejs/node/pull/4729


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>