Add info about layout and bounds to generic (#1207)

* Revise to highlight difference in authoring requirements between generic and presentation
Co-authored-by: Matt King <[email protected]>

index.html

@@ -3403,9 +3403,8 @@ <h2>Definition of Roles</h2>
 			<rdef>generic</rdef>
 			<div class="role-description">
 				<p>A nameless container <a>element</a> that has no semantic meaning on its own.</p>
-				<p>Contrast with <rref>group</rref>, which semantically groups its descendants in a named container.</p>
-				<p>A <code>generic</code> can provide a limited number of accessible states and properties for its descendants, such as <pref>aria-live</pref> attributes. This differentiates it from the <rref>presentation</rref> role.</p>
-				<p>The <code>generic</code> role is intended for implementors of User Agents. Authors SHOULD NOT use this role in content.</p>
+				<p>The <code>generic</code> role is intended for use as the implicit role of generic elements in host languages (such as <abbr title="Hypertext Markup Language">HTML</abbr> <code>div</code> or <code>span</code>), so is primarily for implementors of user agents. Authors SHOULD NOT use this role in content. Authors MAY use <rref>presentation</rref> or <rref>none</rref> to remove implicit accessibility semantics, or a semantic container role such as <rref>group</rref> to semantically group descendants in a named container.</p>
+				<p>Like an element with role <rref>presentation</rref>, an element with role <code>generic</code> can provide a limited number of accessible states and properties for its descendants, such as <pref>aria-live</pref> attributes. However, unlike elements with role <rref>presentation</rref>, <code>generic</code> elements are exposed in <a>accessibility APIs</a> so that assistive technologies can gather certain properties such as layout and bounds.</p>
 			</div>
 			<table class="role-features">
 				<caption>Characteristics:</caption>