src: add comment for TicketKeyCallback

Fixes: #13176
PR-URL: #13193
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

Author: addaleax

src/node_crypto.cc

@@ -1177,6 +1177,8 @@ void SecureContext::SetFreeListLength(const FunctionCallbackInfo<Value>& args) {
 }
 
 
+// Currently, EnableTicketKeyCallback and TicketKeyCallback are only present for
+// the regression test in test/parallel/test-https-resume-after-renew.js.
 void SecureContext::EnableTicketKeyCallback(
     const FunctionCallbackInfo<Value>& args) {
   SecureContext* wrap;