Fix the role attribute definition.

[jyasskin]

@marcoscaceres' #1516 changed https://w3c.github.io/aria/#host_general_role to use a <dfn> for the role attribute, and then @jnurthen's a56ae1818d0933145e6bf0b07c5884580d5f77b5 changed it back. This paragraph claims that "An implementing host language will provide a role attribute", but HTML does not in fact provide one, and @marcoscaceres filed https://github.com/whatwg/html/issues/6890 to have HTML remove its <dfn> for the term.

I don't have a preference for where the attribute is defined for HTML elements, but it should be defined somewhere.

[annevk]

Is it supported on elements outside the HTML, SVG, and MathML namespaces? If so, DOM would probably be the best place.

[jnurthen]

Is it supported on elements outside the HTML, SVG, and MathML namespaces? If so, DOM would probably be the best place.

Theoretically yes - in practice I don't know of any places where it is.