minor symfony#9870 fix(Caution): Validator folder for YAML or XML format (Guikingone, javiereguiluz)

This PR was submitted for the 4.1 branch but it was merged into the 4.0 branch instead (closes symfony#9870).

Discussion
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fix(Caution): Validator folder for YAML or XML format

Hi,

The validator waits for a `config/validator` folder structure if we use the "file" approach, this structure must be respected, it could be a great idea to informat that the structure is mandatory.

Commits
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2f29f7a Reword
bc83185 fix(Caution): Validator folder for YAML or XML format

Author: javiereguiluz

validation.rst

@@ -43,8 +43,9 @@ So far, this is just an ordinary class that serves some purpose inside your
 application. The goal of validation is to tell you if the data
 of an object is valid. For this to work, you'll configure a list of rules
 (called :ref:`constraints <validation-constraints>`) that the object must
-follow in order to be valid. These rules can be specified via a number of
-different formats (YAML, XML, annotations, or PHP).
+follow in order to be valid. These rules are usually defined using PHP code or
+annotations but they can also be defined as a ``validation.yaml`` or
+``validation.xml`` file inside the ``config/validator/`` directory:
 
 For example, to guarantee that the ``$name`` property is not empty, add the
 following: