progressbar: clarify described-by (#1671)

* progressbar: clarify described-by

Resolves #1654

Co-authored-by: Scott O'Hara <scottaohara@users.noreply.github.com>

index.html

@@ -6471,7 +6471,7 @@ <h5>Note regarding the ARIA 1.1 <rref>none</rref> role.</h5>
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 				<p>The author SHOULD supply a <span>value</span> for <pref>aria-valuenow</pref> unless the value is indeterminate, in which case the author SHOULD omit the <pref>aria-valuenow</pref> attribute.
-				Authors SHOULD update this value when the visual progress indicator is updated. If the <code>progressbar</code> is describing the loading progress of a particular region of a page, the author SHOULD use <pref>aria-describedby</pref> to point to the status, and set the <sref>aria-busy</sref> attribute to <code>true</code> on the region until it is finished loading. It is not possible for the user to alter the value of a <code>progressbar</code> because it is always read-only.</p>
+				Authors SHOULD update this value when the visual progress indicator is updated. If the <code>progressbar</code> is describing the loading progress of a particular region of a page, authors SHOULD both use <pref>aria-describedby</pref> to reference the progressbar status, and set the <sref>aria-busy</sref> attribute to <code>true</code> on the region until it is finished loading. It is not possible for the user to alter the value of a <code>progressbar</code> because it is always read-only.</p>
 				<p class="note">Assistive technologies generally will render the value of <pref>aria-valuenow</pref> as a percent of a range between the value of <pref>aria-valuemin</pref> and <pref>aria-valuemax</pref>, unless <pref>aria-valuetext</pref> is specified.</p>
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