Fix #744: clarify control groups and control group owner objects

This attempts to clarify:

- How control groups are created (previously we were just told that
  focusable areas belong to them)
- The relationship between control groups and control group owners

It also removes the redundant distinction between "control group owners"
and "control group owner objects", settling on the latter since it was
more prevalent.

source

@@ -56864,7 +56864,7 @@ dictionary <dfn>RelatedEventInit</dfn> : <span>EventInit</span> {
 
   <ol>
 
-   <li><p>If for some reason <var>subject</var> is not a <span>control group owner</span>
+   <li><p>If for some reason <var>subject</var> is not a <span>control group owner object</span>
    at this point, or if it is <span>inert</span>, abort these steps.</p></li>
 
    <li>
@@ -71298,10 +71298,9 @@ END:VCARD</pre>
   container</span> essentially acts merely as a placeholder for its <span>nested browsing
   context</span>'s <span>active document</span>.</p>
 
-  <p>Each <span>focusable area</span> belongs to a <dfn>control group</dfn>. Each <span>control group</span> has
-  an <dfn data-x="control group owner">owner</dfn>. <span data-x="control group owner">Control group
-  owners</span> are <dfn data-x="control group owner object">control group owner objects</dfn>. The
-  following are <span data-x="control group owner object">control group owner objects</span>:</p>
+  <hr>
+
+  <p>The following create <dfn data-x="control group">control groups</dfn>:
 
   <ul class="brief">
 
@@ -71313,13 +71312,17 @@ END:VCARD</pre>
 
   </ul>
 
-  <p>Each <span>control group owner object</span> owns one <span>control group</span> (though that
-  group might be empty).</p>
+  <p>The corresponding <code>Document</code> object or <code>dialog</code> element is the created
+  <span>control group</span>'s <dfn data-x="control group owner object">owner object</dfn>. (There
+  is always a 1:1 mapping of <span data-x="control group">control groups</span> to <span
+  data-x="control group owner object">control group owner objects</span>.)</p>
 
-  <p>If the <span>DOM anchor</span> of a <span>focusable area</span> is a <span>control group owner
-  object</span>, then that <span>focusable area</span> belongs to that <span>control group owner
-  object</span>'s <span>control group</span>. Otherwise, the <span>focusable area</span> belongs to its
-  <span>DOM anchor</span>'s nearest ancestor <span>control group owner object</span>.</p>
+  <p>Each <span>focusable area</span> belongs to a single <span>control group</span> as follows. If
+  the <span>DOM anchor</span> of a <span>focusable area</span> is a <span>control group
+  owner object</span>, then that <span>focusable area</span> belongs to that <span>control group
+  owner object</span>'s <span>control group</span>. Otherwise, the <span>focusable area</span>
+  belongs to its <span>DOM anchor</span>'s nearest ancestor <span>control group owner
+  object</span>'s <span>control group</span>.</p>
 
   <div class="example">
 
@@ -71333,6 +71336,9 @@ END:VCARD</pre>
 
   </div>
 
+  <p class="note">It is possible for a <span>control group</span> to be empty, i.e. for it to have
+  no <span data-x="focusable area">focusable areas</span>.</p>
+
   <p>An element is <dfn data-x="expressly inert control">expressly inert</dfn> if it is
   <span>inert</span> but it is not a <span>control group owner object</span> and its nearest
   ancestor <span>control group owner object</span> is not <span>inert</span>.</p>
@@ -71443,7 +71449,8 @@ END:VCARD</pre>
 
     <p>Otherwise, if <var>current object</var> is a <span>focusable area</span>, let <var>current
     object</var> be that <span>focusable area</span>'s <span>control group</span>'s <span
-    data-x="control group owner">owner</span>, and return to the step labeled <i>loop</i>.</p>
+    data-x="control group owner object">owner object</span>, and return to the step labeled
+    <i>loop</i>.</p>
 
     <p>Otherwise, if <var>current object</var> is a <code>Document</code> in a <span>nested browsing
     context</span>, let <var>current object</var> be its <span>browsing context container</span>,
@@ -71815,8 +71822,8 @@ END:VCARD</pre>
 
     <p>Otherwise, if <var>old focus target</var> is a <span>focusable area</span>, then let <var>new
     focus target</var> be the first <span>focusable area</span> of its <span>control group</span>
-    (if the <span>control group owner</span> is a <code>Document</code>, this will always be a
-    viewport).</p>
+    (if the <span>control group owner object</span> is a <code>Document</code>, this will always be
+    a viewport).</p>
 
     <p>Otherwise, let <var>new focus target</var> be null.</p>
 
@@ -71999,14 +72006,15 @@ END:VCARD</pre>
 
      <li>
 
-      <p>If <var>entry</var> is a <span>focusable area</span>: Designate <var>entry</var> as the <span>focused area of the control group</span>. If its <span>control
-      group</span>'s <span data-x="control group owner">owner</span> is also a <span>dialog group
+      <p>If <var>entry</var> is a <span>focusable area</span>: Designate <var>entry</var> as the
+      <span>focused area of the control group</span>. If its <span>control group</span>'s <span
+      data-x="control group owner object">owner object</span> is also a <span>dialog group
       manager</span>, then let there be no designated <span data-x="focused dialog of the dialog
       group">focused dialog</span> in that <span>dialog group</span>.</p>
 
       <p class="note">It is possible for <var>entry</var> to be both a <code>dialog</code>
       element and a <span>focusable area</span>, in which case it is its own <span>control group
-      owner</span>.</p>
+      owner object</span>.</p>
 
      </li>