minor symfony#9562 remove part about adding extension (tuanalumi)

This PR was submitted for the 3.4 branch but it was merged into the 2.7 branch instead (closes symfony#9562).

Discussion
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remove part about adding extension

I do not really understand this paragraph and did not follow it, but the `constraints` option still works. So I believe it is something historical.

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Commits
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33446d1 remove part about adding extension

Author: xabbuh

validation.rst

@@ -509,14 +509,6 @@ of the form fields::
         ;
     }
 
-The ``constraints`` option is only available if the ``ValidatorExtension``
-was enabled through the form factory builder::
-
-    Forms::createFormFactoryBuilder()
-        ->addExtension(new ValidatorExtension(Validation::createValidator()))
-        ->getFormFactory()
-    ;
-
 .. index::
    single: Validation; Constraint targets