Change endpoint check from null to document

In whatwg#8886 I made the endpoint
parameter in hide all popovers until never null by making it either a
document or an element, but I forgot to change this null check to a
document check.

Fixes whatwg#8963

source

@@ -82120,8 +82120,8 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
     </ol>
    </li>
 
-   <li><p>If <var>endpoint</var> is null, then run <var>closeAllOpenPopovers</var> and
-   return.</p></li>
+   <li><p>If <var>endpoint</var> is a <code>Document</code>, then run
+   <var>closeAllOpenPopovers</var> and return.</p></li>
 
    <li><p>Let <var>lastToHide</var> be null.</p></li>