Editorial: attribute domintro/index tweaks
* Adds an annotation to prevent "is" and "dir" from getting
over-explained, per https://github.com/whatwg/html-build/pull/210.
* Removes the double description of the disabled attribute
(attr-fe-disabled), once for form elements and once for
form-associated custom elements. There is only one
so there should be only one attribute index entry.