doc: reference worker threads on signal events

PR-URL: #30990
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/process.md

@@ -429,6 +429,8 @@ Signal events will be emitted when the Node.js process receives a signal. Please
 refer to signal(7) for a listing of standard POSIX signal names such as
 `'SIGINT'`, `'SIGHUP'`, etc.
 
+Signals are not available on [`Worker`][] threads.
+
 The signal handler will receive the signal's name (`'SIGINT'`,
  `'SIGTERM'`, etc.) as the first argument.