additional sentence for 204

At the html-a11y tf meeting this morning, I was asked to propose some text to make it clear that authors have control over whether @hidden elements will be exposed to users via accessibility APIs.  It's pretty clear in the existing text, in the 2nd paragraph, that this exposure only applies to elements that are referenced by another element.

<blockquote>
User Agents may thus expose the full semantics of hidden elements to Assistive Technology when appropriate, if such content is referenced indirectly by a unique identifier (ID) reference or valid hash-name reference.
</blockquote>

I propose that we add a note after the 2nd paragraph to clarify the author's control over this.  This text needs some word-smithing, but I think it conveys the idea.

<proposal>
Note:
Authors have control over whether elements with the @hidden attribute will be exposed in this manner.  Only elements that are is referenced indirectly by a unique identifier (ID) reference or valid hash-name reference will be exposed.  If an author does not wish to have a @hidden element exposed, he may achieve this by not referencing the element.
</proposal>

Or something like that...