[cookbook][doctrine] Clarifying how you would practically use the file upload example.

I don't think there is any other way to specify the directory here and stay portable. For all practical purposes (which we want to cover), this seems like the example we should show.

Author: weaverryan

cookbook/doctrine/file_uploads.rst

@@ -55,7 +55,8 @@ First, create a simple Doctrine Entity class to work with::
 
         protected function getUploadRootDir()
         {
-            return '/path/to/uploaded/documents';
+            // the absolute directory path where uploaded documents should be saved
+            return __DIR__.'/../../../../web/uploads/documents';
         }
     }