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>

doc/api/https.markdown

@@ -185,6 +185,7 @@ The following options from [`tls.connect()`][] can also be specified. However, a
 - `secureProtocol`: The SSL method to use, e.g. `SSLv3_method` to force
   SSL version 3. The possible values depend on your installation of
   OpenSSL and are defined in the constant [`SSL_METHODS`][].
+- `servername`: Servername for SNI (Server Name Indication) TLS extension.
 
 In order to specify these options, use a custom [`Agent`][].