v4 backport: 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: #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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)