docs(fetch): added comment on security.

Author: tbosch

fetchLatestSnapshot.php

@@ -6,6 +6,14 @@
   <pre><?php echo `date`; ?></pre>
   <pre><?php
     $ver = $_GET['ver'];
+    <?php
+     /*
+        The url below is the workspace folder for the master build,
+        which gets regenerated on every build and which
+        always contains only one version with a unique name.
+        So this can't be used to rollback to a previous version.
+     */
+    ?>
     $ciBase = 'http://ci.angularjs.org/job/angular.js-angular-master/ws/build/';
     $url = $ciBase.'angular-'.$ver.'.zip';
     if ( !$ver or !preg_match("/^[\.\-\d\w]+$/", $ver) ) {