doc: add cleartextStream.getCipher() in tls

doc/api/tls.markdown

@@ -401,6 +401,17 @@ Example:
 If the peer does not provide a certificate, it returns `null` or an empty
 object.
 
+### cleartextStream.getCipher()
+Returns an object representing the cipher name and the SSL/TLS
+protocol version of the current connection.
+
+Example:
+{ name: 'AES256-SHA', version: 'TLSv1/SSLv3' }
+
+See SSL_CIPHER_get_name() and SSL_CIPHER_get_version() in
+http://www.openssl.org/docs/ssl/ssl.html#DEALING_WITH_CIPHERS for more
+information.
+
 ### cleartextStream.address()
 
 Returns the bound address and port of the underlying socket as reported by the