Updated the characteristics table of "caption" role (#1975)

while reviewing the specs for the caption role, I've realised that the characteristics table, in particular the "Required Accessibility Parent Roles" row, is not fully reflecting the requirements listed above.

    I've updated the "Required Accessibility Parent Roles" table content, adding also "group" and "radiogroup".
    I've also sorted the list of "Required Accessibility Parent Roles" by alphabetical order (both in text and in table).

Closes: #1974

index.html

@@ -1760,11 +1760,11 @@ <h2>Definition of Roles</h2>
 		<div class="role" id="caption">
 			<rdef>caption</rdef>
 			<div class="role-description">
-				<p>Visible content that names, or describes a <rref>group</rref>, <rref>figure</rref>, <rref>table</rref>, <rref>grid</rref>, <rref>radiogroup</rref>, or <rref>treegrid</rref>.</p>
+				<p>Visible content that names, or describes a <rref>figure</rref>, <rref>grid</rref>, <rref>group</rref>, <rref>radiogroup</rref>, <rref>table</rref> or <rref>treegrid</rref>.</p>
 				<p>When using <code>caption</code> authors SHOULD ensure:</p>
 				<ul>
-					<li>The <code>caption</code> is a descendant of a <rref>group</rref>, <rref>figure</rref>, <rref>grid</rref>, <rref>radiogroup</rref>, <rref>table</rref>, or <rref>treegrid</rref>.</li>
-					<li>The <code>caption</code> is the first non-<code>generic</code> descendant of a <rref>group</rref>, <rref>radiogroup</rref>, <rref>grid</rref>, <rref>table</rref>, or <rref>treegrid</rref>.</li>
+					<li>The <code>caption</code> is a descendant of a <rref>figure</rref>, <rref>grid</rref>, <rref>group</rref>, <rref>radiogroup</rref>, <rref>table</rref>, or <rref>treegrid</rref>.</li>
+					<li>The <code>caption</code> is the first non-<code>generic</code> descendant of a <rref>grid</rref>, <rref>group</rref>, <rref>radiogroup</rref>, <rref>table</rref> or <rref>treegrid</rref>.</li>
 					<li>The <code>caption</code> is the first or last non-<code>generic</code> descendant of a <rref>figure</rref>.</li>
 				</ul>
 				<p>If the <code>caption</code> represents an accessible name for its containing element, authors SHOULD specify <pref>aria-labelledby</pref> on the containing element to reference the element with role <code>caption</code>.</p>
@@ -1864,6 +1864,8 @@ <h2>Definition of Roles</h2>
 							<ul>
 								<li><rref>figure</rref></li>
 								<li><rref>grid</rref></li>
+								<li><rref>group</rref></li>
+								<li><rref>radiogroup</rref></li>
 								<li><rref>table</rref></li>
 								<li><rref>treegrid</rref></li>
 							</ul>