Signpost the debate on some script document mismatches

This adds a pointer to #2137, which remains somewhat contested, to the
relevant parts of the prepare and execute algorithms for scripts.

Author: domenic

source

@@ -59211,8 +59211,13 @@ o............A....e
    <li><p>Set the element's <span>preparation-time document</span> to its <span>node
    document</span>.</p>
 
-   <li><p>If <var>parser document</var> is non-null, and <var>parser document</var> is not equal to
-   the element's <span>preparation-time document</span>, then return.</p></li>
+   <li>
+    <p>If <var>parser document</var> is non-null, and <var>parser document</var> is not equal to
+    the element's <span>preparation-time document</span>, then return.</p>
+
+    <p class="XXX">This step is not interoperable and is under debate: see <a
+    href="https://github.com/whatwg/html/issues/2137">issue #2137</a>.</p>
+   </li>
 
    <li id="script-processing-noscript">
 
@@ -59600,9 +59605,14 @@ o............A....e
   <ol>
    <li><p>Let <var>document</var> be <var>scriptElement</var>'s <span>node document</span>.</p></li>
 
-   <li><p>If <var>scriptElement</var>'s <var>parser document</var> is non-null, and
-   <var>scriptElement</var>'s <var>parser document</var> is not equal to <var>document</var>, then
-   return.</p></li>
+   <li>
+    <p>If <var>scriptElement</var>'s <var>parser document</var> is non-null, and
+    <var>scriptElement</var>'s <var>parser document</var> is not equal to <var>document</var>, then
+    return.</p>
+
+    <p class="XXX">This step is not interoperable and is under debate: see <a
+    href="https://github.com/whatwg/html/issues/2137">issue #2137</a>.</p>
+   </li>
 
    <li><p>If <span data-x="concept-script-script">the script's script</span> is null for
    <var>scriptElement</var>, then <span data-x="concept-event-fire">fire an event</span> named <code