minor #10331 (Proposal) Added a note on forms validation (mickaelandrieu, javiereguiluz)

This PR was submitted for the 4.1 branch but it was merged into the 2.8 branch instead (closes #10331).

Discussion
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(Proposal) Added a note on forms validation

Hello!

regarding the way I validate my forms, I've always found that storing validation rules into my entities is a better idea than using the constraints inside my form types.

When I need a really specific control on my form validation, I inject validation groups: do you think it can be viewed as a best practice?

Commits
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68a7931 Reword
21c4c9f Added a note on forms validation

Author: javiereguiluz

best_practices/forms.rst

@@ -145,6 +145,34 @@ view layer:
                class="btn btn-default pull-right" />
     {{ form_end(form) }}
 
+Validation
+----------
+
+The `constraints`_ option allows you to attach `validation constraints`_ to any
+form field. However, doing that prevents the validation from being reused in
+other forms or other places where the mapped object is used.
+
+.. best-practice::
+
+    Do not define your validation constraints in the form but on the object the
+    form is mapped to.
+
+For example, to validate that the title of the post edited with a form is not
+blank, add the following in the ``Post`` object::
+
+    // src/Entity/Post.php
+
+    // ...
+    use Symfony\Component\Validator\Constraints as Assert;
+
+    class Post
+    {
+        /**
+         * @Assert\NotBlank()
+         */
+        public $title;
+    }
+
 Rendering the Form
 ------------------
 
@@ -207,3 +235,6 @@ like the form is *always* processed (even on the GET request).
 ----
 
 Next: :doc:`/best_practices/i18n`
+
+.. _`constraints`: https://symfony.com/doc/current/reference/forms/types/form.html#constraints
+.. _`validation constraints`: https://symfony.com/doc/current/reference/constraints.html