doc: DEFAULT_ECDH_CURVE was added in 0.11.13

Document this with the YAML meta-data.

PR-URL: https://github.com/nodejs/node/pull/10983
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/tls.md

@@ -1077,6 +1077,9 @@ console.log(tls.getCiphers()); // ['AES128-SHA', 'AES256-SHA', ...]
 ```
 
 ## tls.DEFAULT_ECDH_CURVE
+<!-- YAML
+added: v0.11.13
+-->
 
 The default curve name to use for ECDH key agreement in a tls server. The
 default value is `'prime256v1'` (NIST P-256). Consult [RFC 4492] and