minor #12626 Add reference to documentation in FormEvents phpdocs (ifdattic)

This PR was squashed before being merged into the 2.3 branch (closes #12626).

Discussion
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Add reference to documentation in FormEvents phpdocs

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #12512
| License       | MIT
| Doc PR        |

Commits
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2b33ba6 Add reference to documentation in FormEvents phpdocs

Author: fabpot

FormEvents.php

@@ -11,6 +11,12 @@
 namespace Symfony\Component\Form;
 
 /**
+ * To learn more about how form events work check the documentation
+ * entry at {@link http://symfony.com/doc/current/components/form/form_events.html}
+ *
+ * To learn how to dynamically modify forms using events check the cookbook
+ * entry at {@link http://symfony.com/doc/current/cookbook/form/dynamic_form_modification.html}
+ *
  * @author Bernhard Schussek <[email protected]>
  */
 final class FormEvents