doc: clarify synchronous blocking of MessagePort

Fixes: nodejs#25630
Signed-off-by: James M Snell <jasnell@gmail.com>

doc/api/worker_threads.md

@@ -679,6 +679,29 @@ The `ArrayBuffer`s for `Buffer` instances created using
 transferred but doing so renders all other existing views of
 those `ArrayBuffer`s unusable.
 
+#### Synchronous blocking of message handling
+
+The `postMessage()` method will defer the actual transmission and
+handling of the message until after the current synchronous execution
+context completes and the Node.js event loop as allowed to continue
+turning.
+
+In the following example, for instance, the `onmessage` handler for
+`mc.port1` will not be invoked until after the `for` loop completes
+and the Node.js event loop is not blocked.
+
+```js
+const mc = new MessageChannel();
+
+mc.port1.onmessage = console.log;
+
+mc.port2.postMessage(1);
+
+for (let n = 0; n < 1e9; n++) {
+  console.log(n);
+}
+```
+
 #### Considerations when cloning objects with prototypes, classes, and accessors
 
 Because object cloning uses the [HTML structured clone algorithm][],