Remove incorrect statement about the disabled attribute on <form> (#35713)

Remove incorrect statement about the disabled attribute on <form> (#35713)

> You can disable every form element within a form with the `disabled` attribute on the `<form>`.

I really want to be mistaken, because this would be a very useful feature! But I don't believe it's true. I can't find anything about this on MDN Web Docs, and adding the `disabled` attribute to a `<form>` does nothing on any browser in my testing.

The `disabled` attribute on a `<fieldset>` does disable all descendant form controls – perhaps that's where the mixup has come from.