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 <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: sam-github

doc/api/tls.md

@@ -898,6 +898,15 @@ 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