Throw InvalidNodeTypeError for doctype more eagerly

This matches how major engines behave and is covered by multiple
tests in WPT already.

Closes #118.

index.html

@@ -465,6 +465,9 @@ <h2>
             <li>If <var>node</var> is null, this method must behave identically
             as <code>removeAllRanges()</code> and abort these steps.
             </li>
+            <li>If <var>node</var> is a {{DocumentType}}, throw an
+                {{InvalidNodeTypeError}} exception and abort these steps.
+            </li>
             <li>The method must throw an {{IndexSizeError}} exception if <var>
               offset</var> is longer than <var>node</var>'s [=Node/length=] and
               abort these steps.
@@ -625,6 +628,9 @@ <h2>
             The method must follow these steps:
           </p>
           <ol>
+            <li>If <var>node</var> is a {{DocumentType}}, throw an
+                {{InvalidNodeTypeError}} exception and abort these steps.
+            </li>
             <li>If <var>node</var>'s [=tree/root=] is not the <a>document</a>
             associated with [=this=], abort these steps.
             </li>