minor #3419 doctrine file upload example uses dir -- caution added (c…

…ordoval)

This PR was merged into the 2.3 branch.

Discussion
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doctrine file upload example uses dir -- caution added

| Q             | A                   |
| ---           | ---                 |
| Doc fix?      | yes                 |
| New docs?     | no                  |
| Applies to    | 2.3+                |
| Fixed tickets | #3363               |
| License       | CC-ASA 3.0 Unported |

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Commits
-------

1620ed8 3363-doctrine-file-upload-example-uses-dir

cookbook/doctrine/file_uploads.rst

@@ -300,6 +300,15 @@ object, which is what's returned after a ``file`` field is submitted::
 Using Lifecycle Callbacks
 -------------------------
 
+.. caution::
+
+    Using lifecycle callbacks is a limited technique that has some drawbacks.
+    If you want to remove the hard coded ``__DIR__`` reference inside
+    the ``Document::getUploadRootDir()`` method, the best way is to start
+    using explicit :doc:`doctrine listeners </cookbook/doctrine/event_listeners_subscribers>`
+    where you will be able to inject kernel parameters such as
+    ``kernel.root_dir`` to be able to build absolute paths.
+
 Even if this implementation works, it suffers from a major flaw: What if there
 is a problem when the entity is persisted? The file would have already moved
 to its final location even though the entity's ``path`` property didn't