Clarify that there is no attribute named "inert"

Fixes http://w3bug.com/28906

source

@@ -71972,6 +71972,10 @@ END:VCARD</pre>
 
   <h3>Inert subtrees</h3>
 
+  <p class="note">This section <strong>does not</strong> define or create any content attribute
+  named "inert". This section merely defines an abstract <em>concept</em> of
+  <span data-x="inert">inertness</span>.</p>
+
   <p>A node (in particular elements and text nodes) can be marked as <dfn>inert</dfn>. When a node
   is <span>inert</span>, then the user agent must act as if the node was absent for the purposes of
   targeting user interaction events, may ignore the node for the purposes of text search user
@@ -116553,6 +116557,7 @@ INSERT INTERFACES HERE
   Chris Morris,
   Chris Pearce,
   Chris Peterson,
+  Chris Rebert,
   Chris Weber,
   Christian Biesinger,
   Christian Johansen,