resolves #1256

This PR modifies the previous note about aria-hidden=false to instead indicate that as of ARIA 1.3 is serves as a synonym for the 'undefined' value.

It also includes a minor editorial change to remove the mention of 'screen readers' from the last paragraph of the normative text.  It seemed to state, especially when the sentence ends with referring to 'assistive technologies' in general.

See [last comment](#1256 (comment))

Chrome issue: https://bugs.chromium.org/p/chromium/issues/detail?id=1500299

Question: assuming this PR is approved, would we also want to add the following sentence to the note?
>In future versions of ARIA, the undefined value will be deprecated in favor of the false value.

Co-authored-by: James Craig <cookiecrook@users.noreply.github.com>