minor #6251 To use annotations, files must be removed (pbowyer)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #6251).

Discussion
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To use annotations, files must be removed

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | Tested on 3.0
| Fixed tickets | N/A

I'm proposing this change after spending an hour trying to work out why my Repository class wasn't loading.

Thanks to `ysor123` on IRC, we diagnosed the problem was the XML mapping files generated when reverse engineering my database, which I hadn't deleted.

This is obliquely referred to at http://symfony.com/doc/current/book/doctrine.html#add-mapping-information:

> A bundle can accept only one metadata definition format. For example, it's not possible to mix YAML metadata definitions with annotated PHP entity class definitions.

It'd be great to clarify this for future readers.

Commits
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06de6c0 Updated per xabbuh's comment
6a526bf To use annotations, files must be removed

Author: weaverryan

cookbook/doctrine/reverse_engineering.rst

@@ -101,8 +101,9 @@ execute the second command only.
 
 .. tip::
 
-    If you want to use annotations, you can safely delete the XML (or YAML) files
-    after running these two commands.
+    If you want to use annotations, you must remove the XML (or YAML) files
+    after running these two commands. This is necessary as
+    :ref:`it is not possible to mix mapping configuration formats <book-doctrine-adding-mapping>`
 
 For example, the newly created ``BlogComment`` entity class looks as follow::