Add a note explaining transient observers. Fixes https://www.w3.org/B…

…ugs/Public/show_bug.cgi?id=27713

dom.bs

@@ -2475,9 +2475,13 @@ A <dfn export for=MutationObserver>registered observer</dfn> consists of an <b>o
 <a>registered observer</a> that has a <b>source</b> which is a
 <a>registered observer</a>.
 
+<p class="note no-backref"><a>Transient registered observers</a> are used to track
+mutations within a given <a>node</a>'s <a>descendants</a> after <a>node</a> has been
+removed so they do not get lost when <code>subtree</code> is set to true on <a>node</a>'s
+<a>parent</a>.
+
 
-<h4 id='interface-mutationobserver'>
-Interface {{MutationObserver}}</h4>
+<h4 id=interface-mutationobserver>Interface {{MutationObserver}}</h4>
 
 <pre class="idl">
 [Constructor(MutationCallback callback)]

dom.html

@@ -187,8 +187,7 @@ <h2 class="no-num no-toc no-ref heading settled" id="contents"><span class="cont
       <li><a href="#mutation-observers"><span class="secno">4.3</span> <span class="content">
 Mutation observers</span></a>
        <ul class="toc">
-        <li><a href="#interface-mutationobserver"><span class="secno">4.3.1</span> <span class="content">
-Interface <code class="idl"><span>MutationObserver</span></code></span></a>
+        <li><a href="#interface-mutationobserver"><span class="secno">4.3.1</span> <span class="content">Interface <code class="idl"><span>MutationObserver</span></code></span></a>
         <li><a href="#queueing-a-mutation-record"><span class="secno">4.3.2</span> <span class="content">
 Queuing a mutation record</span></a>
         <li><a href="#interface-mutationrecord"><span class="secno">4.3.3</span> <span class="content">
@@ -3282,9 +3281,14 @@ <h3 class="heading settled" data-level="4.3" id="mutation-observers"><span class
 <a data-link-type="dfn" href="#registered-observer">registered observer</a>.</p>
 
 
+   <p class="note no-backref" role="note"><a data-link-type="dfn" href="#transient-registered-observer">Transient registered observers</a> are used to track
+mutations within a given <a data-link-type="dfn" href="#concept-node">node</a>’s <a data-link-type="dfn" href="#concept-tree-descendant">descendants</a> after <a data-link-type="dfn" href="#concept-node">node</a> has been
+removed so they do not get lost when <code>subtree</code> is set to true on <a data-link-type="dfn" href="#concept-node">node</a>’s
+<a data-link-type="dfn" href="#concept-tree-parent">parent</a>.
 
-   <h4 class="heading settled" data-level="4.3.1" id="interface-mutationobserver"><span class="secno">4.3.1. </span><span class="content">
-Interface <code class="idl"><a data-link-type="idl" href="#mutationobserver">MutationObserver</a></code></span><a class="self-link" href="#interface-mutationobserver"></a></h4>
+
+</p>
+   <h4 class="heading settled" data-level="4.3.1" id="interface-mutationobserver"><span class="secno">4.3.1. </span><span class="content">Interface <code class="idl"><a data-link-type="idl" href="#mutationobserver">MutationObserver</a></code></span><a class="self-link" href="#interface-mutationobserver"></a></h4>
 
 
    <pre class="idl">[<a class="idl-code" data-link-type="constructor" href="#dom-mutationobserver-mutationobserver">Constructor</a>(<a data-link-type="idl-name" href="#callbackdef-mutationcallback">MutationCallback</a> callback)]