Clarify that modal dialogs escape inertness of ancestors

Closes whatwg#7808.

source

@@ -75962,9 +75962,12 @@ END:VCARD</pre>
   <var>document</var>'s <span>top layer</span>. While <var>document</var> is so blocked, every node
   that is <span>connected</span> to <var>document</var>, with the exception of the
   <var>subject</var> element and its <span>flat tree</span> descendants, must become
-  <span>inert</span>. (The elements excepted by this paragraph can additionally become
-  <span>inert</span> through other means; being part of a modal dialog does not "protect" a node
-  from being <span>inert</span>.)</p>
+  <span>inert</span>.</p>
+
+  <p><var>subject</var> can additionally become <span>inert</span> via the <code
+  data-x="attr-inert">inert</code> attribute, but only if specified on <var>subject</var> itself
+  (i.e., <var>subject</var> escapes inertness of ancestors); <var>subject</var>'s <span>flat
+  tree</span> descendants can become <span>inert</span> in a similar fashion.</p>
 
   <p class="note">The <code>dialog</code> element's <code
   data-x="dom-dialog-showModal">showModal()</code> method causes this mechanism to trigger, by
@@ -75974,8 +75977,9 @@ END:VCARD</pre>
   <h4>The <dfn data-x="attr-inert"><code>inert</code></dfn> attribute</h4>
 
   <p>The <code data-x="attr-inert">inert</code> attribute is a <span>boolean attribute</span> that
-  indicates, by its presence, that the element and all its <span>flat tree</span> descendants are
-  to be made <span>inert</span> by the user agent.</p>
+  indicates, by its presence, that the element and all its <span>flat tree</span> descendants which
+  don't otherwise escape inertness (such as modal dialogs) are to be made <span>inert</span> by the
+  user agent.</p>
 
   <p class="note">By default, there is no persistent visual indication of a subtree being inert.
   Authors are encouraged to clearly mark what parts of their document are active and which are