Add guidance for header/footer mappings when used with ARIA roles

[rahimabdi]

ARIA introduced new sectionheader/sectionfooter roles for <header> and <footer>, respectively: w3c/aria#1931. And when <header>/<footer> are descendants of native HTML sectioning elements (e.g., <main>), their sectionheader/sectionfooter mappings are also addressed in this html-aam issue: #585.

However, it's currently unclear how <header>/<footer> should be exposed when they are descendants of ARIA equivalents for sectioning elements, e.g.,:

<div role="main">
    <header aria-label="header">...</header>  <!-- Should this be exposed as role of banner, generic or new sectionheader? -->
    <footer aria-label="footer">...</footer> <!-- Should this be exposed as role of contentinfo, generic or new sectionfooter? -->
</div>

html-aam should specify treatment of these two elements when used with ARIA roles. After discussion with @scottaohara, it would make sense to align with how <header>/<footer> are treated when they are descendants of other native sectioning elements; this also aligns with priority of constituencies for users who would benefit from section header/section footer semantics when authors use ARIA.

CC @scottaohara