Update comments

Author: tinayuangao

tools/gulp/task_helpers.ts

@@ -208,7 +208,10 @@ export function isTravisPushBuild() {
   return process.env['TRAVIS_PULL_REQUEST'] === 'false';
 }
 
-/** Open Google Cloud Storage for screenshots */
+/**
+ * Open Google Cloud Storage for screenshots.
+ * The files uploaded to google cloud are also available to firebase storage.
+ */
 export function openScreenshotsBucket() {
   let gcs = gcloud.storage({
     projectId: 'material2-screenshots',

tools/gulp/tasks/screenshots.ts

@@ -139,7 +139,10 @@ function diffScreenshot(filename: string, database: admin.database.Database,
   }
 }
 
-/** Upload a list of filenames to firebase database as gold. */
+/**
+ * Upload a list of filenames to firebase database as gold.
+ * This is necessary for control panel since google-cloud is not available to client side.
+ */
 function setScreenFilenames(database: admin.database.Database,
                             prNumber?: string) {
   let filenames: string[] = getLocalScreenshotFiles(SCREENSHOT_DIR);