Footer/sectionfooter title and html spec description is ambiguous

[giacomo-petri]

While reviewing #2543, I realised that in HTML AAM the footer title and HTML spec desc (https://www.w3.org/TR/html-aam-1.0/#el-footer) is quite ambiguous:

<h4 id="el-footer">footer(scoped to themain element,a sectioning content element)</h4>

and

<td><a data-cite="HTML">footer</a> (scoped to the <a data-cite="HTML">main</a> element, a <a data-cite="HTML/dom.html#sectioning-content">sectioning content</a> element)</td>

As written, it suggests that the main element is a sectioning content element, while it would be clearer to say: "main element, or a sectioning content element".