Adding a caution to the getUploadRootDir() method

| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | symfony#4177 [Doctrine] Security issue in handling file uploads with Doctrine

cookbook/doctrine/file_uploads.rst

@@ -99,6 +99,13 @@ file.
     If you're using annotations to specify your validation rules (as shown
     in this example), be sure that you've enabled validation by annotation
     (see :ref:`validation configuration <book-validation-configuration>`).
+
+.. caution::
+
+   If you use the getUploadRootDir() method, be aware that this will save 
+   the file inside the document root, which can be accessed by everyone. 
+   Consider placing it out of the document root and adding custom viewing 
+   logic when you need to secure the files.
 
 To handle the actual file upload in the form, use a "virtual" ``file`` field.
 For example, if you're building your form directly in a controller, it might