javascript: add some comments to the upload systems

app/javascript/shared/activestorage/ujs.js

@@ -27,6 +27,8 @@ addUploadEventListener(INITIALIZE_EVENT, ({ target, detail: { id, file } }) => {
 
 addUploadEventListener(START_EVENT, ({ target, detail: { id } }) => {
   ProgressBar.start(id);
+  // At the end of the upload, the form will be submitted again.
+  // Avoid the confirm dialog to be presented again then.
   const button = target.form.querySelector('button.primary');
   if (button) {
     button.removeAttribute('data-confirm');

app/javascript/shared/activestorage/uploader.js

@@ -3,7 +3,7 @@ import ProgressBar from './progress-bar';
 
 /**
   Uploader class is a delegate for DirectUpload instance
-  used to track lifecycle and progress of un upload.
+  used to track lifecycle and progress of an upload.
   */
 export default class Uploader {
   constructor(input, file, directUploadUrl) {

app/javascript/shared/remote-poller.js

@@ -29,6 +29,13 @@ delegate('click', '[data-attachment-refresh]', event => {
   attachementPoller.check();
 });
 
+// Periodically check the state of a set of URLs.
+//
+// Each time the given URL is requested, the matching `show.js.erb` view is rendered,
+// causing the state to be refreshed.
+//
+// This is used mainly to refresh attachments during the anti-virus check,
+// but also to refresh the state of a pending spreadsheet export.
 class RemotePoller {
   urls = new Set();
   timeout;