H1-H6 AND ROLE=HEADING allowed descendants differ

[stevefaulkner]

h1-h6 allows phrasing content
https://html.spec.whatwg.org/multipage/dom.html#phrasing-content-2

aria in html role=heading allows

Flow content but with no main element, heading content, sectioning content, or sectioning roots descendants.

https://www.w3.org/TR/html-aria/#allowed-descendants-of-aria-roles

which results in less than optimal markup patterns being allowed. refer to validator/validator#1282 as an example. I suggest the allowed descendants rule for role=heading be tightened up.

[scottaohara]

related to the work of #362 to make these normative requirements