Change aria-hidden=false to synonym of undefined

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.

index.html

@@ -11853,9 +11853,9 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
 			<div class="state-description">
 				<p><a>Indicates</a> whether the <a>element</a> is exposed to an accessibility API. See related <sref>aria-disabled</sref>.</p>
 				<p>User agents determine an element's [=element/hidden=] status based on whether it is rendered, and the rendering is usually controlled by CSS. For example, an element whose <code>display</code> property is set to <code>none</code> is not rendered. An element is considered [=element/hidden=] if it, or any of its ancestors are not rendered or have their <code>aria-hidden</code> attribute value set to <code>true</code>.</p>
-				<p>Authors MAY, with caution, use aria-hidden to hide visibly rendered content from assistive technologies <em>only</em> if the act of hiding this content is intended to improve the experience for users of assistive technologies by removing redundant or extraneous content. Authors using aria-hidden to hide visible content from screen readers MUST ensure that identical or equivalent meaning and functionality is exposed to assistive technologies.</p>
+				<p>Authors MAY, with caution, use aria-hidden to hide visibly rendered content from assistive technologies <em>only</em> if the act of hiding this content is intended to improve the experience for users of assistive technologies by removing redundant or extraneous content. Authors using aria-hidden to hide visible content MUST ensure that identical or equivalent meaning and functionality is exposed to assistive technologies.</p>
 				<p class="note">Authors are advised to use extreme caution and consider a wide range of disabilities when hiding visibly rendered content from assistive technologies. For example, a sighted, dexterity-impaired individual might use voice-controlled assistive technologies to access a visual interface. If an author hides visible link text "Go to checkout" and exposes similar, yet non-identical link text "Check out now" to the accessibility API, the user might be unable to access the interface they perceive using voice control. Similar problems can also arise for screen reader users. For example, a sighted telephone support technician might attempt to have the blind screen reader user click the "Go to checkout" link, which they might be unable to find using a type-ahead item search ("Go to…").</p>
-				<p class="note">At the time of this writing, <code><sref>aria-hidden</sref>="false"</code> is known to work inconsistently in browsers. As future implementations improve, use caution and test thoroughly before relying on this approach.</p>
+				<p class="note">As of ARIA 1.3, due to inconsistent support in browsers, and the pervasiveness of user interfaces that rely on assuming <code><sref>aria-hidden</sref>="false"</code> does not expose hidden content to the accessibility tree, the <code>false</code> value is now synonymous with <code>undefined</code>.</p>
 			</div>
 			<table class="state-features">
 				<caption>Characteristics:</caption>
@@ -11895,7 +11895,7 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
 				<tbody>
 					<tr>
 						<th class="value-name" scope="row">false</th>
-						<td class="value-description">The element is exposed to the accessibility API as if it was rendered.</td>
+						<td class="value-description">The element's [=element/hidden=] state is determined by the user agent based on whether it is rendered. Synonym of <code>undefined</code>.</td>
 					</tr>
 					<tr>
 						<th class="value-name" scope="row">true</th>