doc: add TLSSocket.isSessionReused() docs

The API has existed forever and is used in our unit tests. It is
supported for TLS1.3 as well as 1.2 and useful for troubleshooting, so
it should be documented.

PR-URL: #25423
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/tls.md

@@ -811,6 +811,15 @@ It may be useful for debugging.
 
 See [Session Resumption][] for more information.
 
+### tlsSocket.isSessionReused()
+<!-- YAML
+added: v0.5.6
+-->
+
+* Returns: {boolean} `true` if the session was reused, `false` otherwise.
+
+See [Session Resumption][] for more information.
+
 ### tlsSocket.localAddress
 <!-- YAML
 added: v0.11.4