Upgrade an existing transport-based connection to TLS.
- Return a new transport instance, that the *protocol* must start using
- immediately after the *await*. The *transport* instance passed to
- the *start_tls* method should never be used again.
+ Create a TLS coder/decoder instance and insert it between the *transport*
+ and the *protocol*. The coder/decoder implements both *transport*-facing
+ protocol and *protocol*-facing transport.
+
+ Return the created two-interface instance. After *await*, the *protocol*
+ must stop using the original *transport* and communicate with the returned
+ object only because the coder caches *protocol*-side data and sporadically
+ exchanges extra TLS session packets with *transport*.
Parameters: