minor #15017 [Translation][Validator] Mention symfony/translation (wkania)

This PR was merged into the 4.4 branch.

Discussion
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[Translation][Validator] Mention symfony/translation

First, when I was checking the PHP attributes syntax for the message, I encounter a problem that the message is not translated.
There was no error. The problem was I didn't install `symfony/translation ` . So maybe we should mention this?

Second the text:

> Now, create a ``validators`` catalog file in the ``translations/`` directory:

inform us to create a structure like `translations/validators` but the commented path in the examples is different.

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Commits
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c4a398e [Validator][Translation] Mention symfony/translation

Author: javiereguiluz

validation/translations.rst

@@ -8,6 +8,11 @@ If you're using validation constraints with the Form component, you can translat
 the error messages by creating a translation resource for the
 ``validators`` :ref:`domain <translation-resource-locations>`.
 
+.. note::
+
+    In order to translate the error message, you should have installed the
+    symfony/translation component with Composer.
+
 To start, suppose you've created a plain-old-PHP object that you need to
 use somewhere in your application::
 
@@ -93,7 +98,7 @@ Now, create a ``validators`` catalog file in the ``translations/`` directory:
 
     .. code-block:: xml
 
-        <!-- translations/validators.en.xlf -->
+        <!-- translations/validators/validators.en.xlf -->
         <?xml version="1.0"?>
         <xliff version="1.2" xmlns="urn:oasis:names:tc:xliff:document:1.2">
             <file source-language="en" datatype="plaintext" original="file.ext">
@@ -108,12 +113,12 @@ Now, create a ``validators`` catalog file in the ``translations/`` directory:
 
     .. code-block:: yaml
 
-        # translations/validators.en.yaml
+        # translations/validators/validators.en.yaml
         author.name.not_blank: Please enter an author name.
 
     .. code-block:: php
 
-        // translations/validators.en.php
+        // translations/validators/validators.en.php
         return [
             'author.name.not_blank' => 'Please enter an author name.',
         ];