doc: minor cleanup of tls.getProtocol()

Improve markup and return value description. PR-URL: #24533 Reviewed-By: Colin Ihrig <[email protected]> Reviewed-By: Vse Mozhet Byt <[email protected]>

Author: sam-github

doc/api/tls.md

@@ -669,19 +669,19 @@ 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.