doc: minor cleanup of tls.getProtocol()

Improve markup and return value description.

PR-URL: #24533
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

doc/api/tls.md

@@ -756,19 +756,19 @@ to implement the `tls-unique` channel binding from [RFC 5929][].
 added: v5.7.0
 -->
 
-* Returns: {string}
+* Returns: {string|null}
 
 Returns a string containing the negotiated SSL/TLS protocol version of the
 current connection. The value `'unknown'` will be returned for connected
 sockets that have not completed the handshaking process. The value `null` will
 be returned for server sockets or disconnected client sockets.
 
-Example responses include:
+Protocol versions are:
 
-* `TLSv1`
-* `TLSv1.1`
-* `TLSv1.2`
-* `unknown`
+* `'TLSv1'`
+* `'TLSv1.1'`
+* `'TLSv1.2'`
+* `'SSLv3'`
 
 See <https://www.openssl.org/docs/man1.1.0/ssl/SSL_get_version.html> for more
 information.