Disclosure pattern: Distinguish the widget from the control (pull #1880)

Revises intro of disclosure pattern to clarify that a disclosure widget has two parts, a button and section of content.

Co-authored-by: Matt King <a11yThinker@Gmail.com>

aria-practices.html

@@ -1218,8 +1218,9 @@ <h4>WAI-ARIA Roles, States, and Properties</h4>
     <section class="widget" id="disclosure">
       <h3>Disclosure (Show/Hide)</h3>
       <p>
-        A disclosure is a <a href="#button">button</a> that controls visibility of a section of content.
-        When the controlled content is hidden, it is often styled as a typical push button with a right-pointing arrow or triangle to hint that activating the button will display additional content.
+        A disclosure is a widget that enables content to be either collapsed (hidden) or expanded (visible).
+        It has two elements: a disclosure <a href="#button">button</a> and a section of content whose visibility is controlled by the button.
+        When the controlled content is hidden, the button is often styled as a typical push button with a right-pointing arrow or triangle to hint that activating the button will display additional content.
         When the content is visible, the arrow or triangle typically points down.
       </p>