[doc] Impossible to load asynchronously

[nytope]

The documentation says that to load CKEditor asynchronously, we need "form_javascript" function from IvoryFormExtraBundle. However, IvoryFormExtraBundle is not compatible with symfony 4. Is there another way to load CKEditor asynchronously, or de we have to remove this feature ?

Edit : It is possible to load CKEditor asynchronously if we just create "form_javascript" twig function ourself. Do you want me to update the documentation ?