Tweak "debug on start" description in chrome://serviceworker-internals

Some users of the checkbox reported not expecting the SW to pause on startup.

BUG=455708

Review URL: https://codereview.chromium.org/939763003

Cr-Commit-Position: refs/heads/master@{#317009}

content/browser/resources/service_worker/serviceworker_internals.html

@@ -126,7 +126,7 @@
                 jsvalues=".checked:$this.debug_on_start">
           </span>
           <span>
-            Opens the DevTools window for ServiceWorker on start for debugging.
+            Open DevTools window and pause JavaScript execution on Service Worker startup for debugging.
           </span>
         </label>
       </div>