doc: add tls.DEFAULT_ECDH_CURVE
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc
Description of change
A user can change the default curve for ECDH key agreement by using tls.DEFAULT_ECDH_CURVE.
From https://github.com/nodejs/node/issues/1495#issuecomment-95536777, forward-port 02a51cf to master.
/to @shigeki