doc: add comments to empty blocks in worker_threads text

PR-URL: https://github.com/nodejs/node/pull/41831
Refs: https://eslint.org/docs/rules/no-empty
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/worker_threads.md

@@ -1267,7 +1267,9 @@ import {
 
 if (isMainThread) {
   new Worker(new URL(import.meta.url));
-  for (let n = 0; n < 1e10; n++) {}
+  for (let n = 0; n < 1e10; n++) {
+    // Looping to simulate work.
+  }
 } else {
   // This output will be blocked by the for loop in the main thread.
   console.log('foo');
@@ -1284,7 +1286,9 @@ const {
 
 if (isMainThread) {
   new Worker(__filename);
-  for (let n = 0; n < 1e10; n++) {}
+  for (let n = 0; n < 1e10; n++) {
+    // Looping to simulate work.
+  }
 } else {
   // This output will be blocked by the for loop in the main thread.
   console.log('foo');