minor #6305 Mention IvoryCKEditorBundle in the Symfony Forms doc (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #6305).

Discussion
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Mention IvoryCKEditorBundle in the Symfony Forms doc

This is the most obvious mention we can do. If you have other ideas to mention this bundle, please share them. Thanks!

Commits
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6126711 Mention IvoryCKEditorBundle in the Symfony Forms doc

Author: xabbuh

reference/forms/types/textarea.rst

@@ -28,6 +28,13 @@ Renders a ``textarea`` HTML element.
 | Class       | :class:`Symfony\\Component\\Form\\Extension\\Core\\Type\\TextareaType` |
 +-------------+------------------------------------------------------------------------+
 
+.. tip::
+
+    If you prefer to use an **advanced WYSIWYG editor** instead of a plain
+    textarea, consider using the IvoryCKEditorBundle community bundle. Read
+    `its documentation <https://symfony.com/doc/current/bundles/IvoryCKEditorBundle/index.html>`_
+    to learn how to integrate it in your Symfony application.
+
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