Clarify that the HTML fragment serialisation algorithm does not throw

The part of the algorithm that could throw was commented out in commit
9219160.

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=28113

PR: whatwg#631

source

@@ -105643,8 +105643,7 @@ document.body.appendChild(text);
 
   <p>The following steps form the <dfn>HTML fragment serialisation algorithm</dfn>. The algorithm
   takes as input a DOM <code>Element</code>, <code>Document</code>, or <code>DocumentFragment</code>
-  referred to as <var>the node</var>, and either returns a string or throws an
-  exception.</p>
+  referred to as <var>the node</var>, and returns a string.</p>
 
   <p class="note">This algorithm serialises the <em>children</em> of the node being serialised, not
   the node itself.</p>
@@ -105829,12 +105828,6 @@ document.body.appendChild(text);
 
       </dl>
 
-      <!--
-      <p>Other node types (e.g. <code data-x="">Attr</code>) cannot occur as children of elements.
-      If, despite this, they somehow do occur, this algorithm must throw an
-      <code>InvalidStateError</code> exception.</p>
-      -->
-
      </li>
 
     </ol>