doc: suggest worker threads in cluster docs

The current documentation is incorrect in that it says "A single
instance of Node.js runs in a single thread," which is not true due
to the addition of worker threads.

This patch removes the incorrect statement and instead suggests that
applications consider using worker threads when process isolation is
not needed.

Refs: nodejs#20876

Author: tniessen

doc/api/cluster.md

@@ -6,9 +6,11 @@
 
 <!-- source_link=lib/cluster.js -->
 
-A single instance of Node.js runs in a single thread. To take advantage of
-multi-core systems, the user will sometimes want to launch a cluster of Node.js
-processes to handle the load.
+Clusters of Node.js processes can be used to run multiple instances of Node.js
+that can distribute workloads among their application threads. When process
+isolation is not needed, the newer [worker threads][] module can be used
+instead, which allows running multiple application threads within a single
+Node.js instance.
 
 The cluster module allows easy creation of child processes that all share
 server ports.
@@ -1122,3 +1124,4 @@ socket.on('data', (id) => {
 [`process` event: `'message'`]: process.md#event-message
 [`server.close()`]: net.md#event-close
 [`worker.exitedAfterDisconnect`]: #workerexitedafterdisconnect
+[worker threads]: worker_threads.md