Bring display-mode: picture-in-picture back (cherry-picks w3c#9920).

mediaqueries-5/Overview.bs

@@ -1343,6 +1343,16 @@ Display Modes: the ''display-mode'' media feature </h3>
 			that are not fullscreen.
 
 			Corresponds to the [=display mode/browser=] display mode.
+
+		<dt><dfn>picture-in-picture</dfn>
+		<dd>
+			This mode allows users to continue consuming media while they interact
+			with other sites or applications on their device.
+			The web application is displayed in a floating and always-on-top window.
+			A user agent may include other platform specific UI elements,
+			such as "back-to-tab" and "site information" buttons
+			or whatever is customary on the platform and user agent.
+
 	</dl>
 
 	<div class="example">