doc: add tls.DEFAULT_ECDH_CURVE

A user can change the default curve for ECDH key agreement by
using tls.DEFAULT_ECDH_CURVE.

PR-URL: https://github.com/nodejs/node/pull/10264


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>