Add missing description for the allow-downloads keyword

Closes #6411.

source

@@ -30460,8 +30460,9 @@ interface <dfn>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
   data-x="attr-iframe-sandbox-allow-popups-to-escape-sandbox">allow-popups-to-escape-sandbox</code>
   keywords re-enable forms, modal dialogs, screen orientation lock, the pointer lock API, popups,
   the presentation API, scripts, and the creation of unsandboxed <span data-x="auxiliary browsing
-  context">auxiliary browsing contexts</span> respectively. <ref spec=POINTERLOCK>
-  <ref spec=SCREENORIENTATION> <ref spec=PRESENTATION></p>
+  context">auxiliary browsing contexts</span> respectively. The <code
+  data-x="attr-iframe-sandbox-allow-downloads">allow-downloads</code> keyword allows content to
+  perform downloads. <ref spec=POINTERLOCK> <ref spec=SCREENORIENTATION> <ref spec=PRESENTATION></p>
 
   <p>The <code
   data-x="attr-iframe-sandbox-allow-top-navigation">allow-top-navigation</code> and <code