This PR closes #1765 and is related to work that was done in #2037, but scoped only to the original issue I filed.

The intent of this PR is to identify not only how user agents would need to handle focusable elements that are aria-hidden (explicitly or due to being a descendant of an aria-hidden container) - but for the case where a focusable element is within an aria-hidden container, that the entire subtree would need to be re-exposed so that any other relevant information to the user could be made available.  (e.g., so as to not just expose a "learn more" link, with no way to determine what someone would be learning about)

a simple example being like:

```
<div aria-hidden=true>
  <h3>Something or other</h3>
   some details about said something, or other.
  <a href=#>Learn more!</a>
</div>
```

pulling in aaron's suggested edits

adds in a reference to the tree exclusion section, and updates bits of content here to include reference to html's `inert` attribute, and to try and acknowledge the change to aria-hidden=false by also referencing aria-hidden=undefined.

this has nothing to do with my PR, but there is an errant closing `</p>` tag after a list in the progressbar section. 

making this change here to get rid of the error