doc: improve TLS/SSL introduction

Author: tniessen

doc/api/tls.md

@@ -16,7 +16,8 @@ const tls = require('tls');
 
 ## TLS/SSL concepts
 
-The TLS/SSL is a public/private key infrastructure (PKI). For most common
+TLS/SSL is a set of protocols that rely on a public key infrastructure (PKI) to
+enable secure communication between a client and a server. For most common
 cases, each server must have a _private key_.
 
 Private keys can be generated in multiple ways. The example below illustrates