Clarify metageneration comments (#1005)

Author: Ace Nassri

functions/helloworld/index.js

@@ -98,7 +98,7 @@ exports.helloGCS = (event, callback) => {
     console.log(`File ${file.name} deleted.`);
   } else if (file.metageneration === '1') {
     // metageneration attribute is updated on metadata changes.
-    // on create value is 1
+    // value is 1 if file was newly created or overwritten
     console.log(`File ${file.name} uploaded.`);
   } else {
     console.log(`File ${file.name} metadata updated.`);