Tweak language around modal dialogs (#4381)

Closes #4185

Note that this only softens/expands some of the example language in the
understanding documents. The original issue #4185 seems to conflate
non-normative examples in a non-normative document with requirements of
how modal dialogs should be implemented ... they are not. They provide
examples of appropriate focus order and absence of keyboard traps. They
are not the only ways in which focus order or absence of keyboard traps
can be achieved. Examples are non-exhaustive, and there is no need - in
my view - to expand these further to specifically address variations
that result from different behaviours related to `<dialog>`

---------

Co-authored-by: Adam Page <adam@adampage.net>
Co-authored-by: Mike Gower <mikegower@gmail.com>

Author: 3 people

understanding/20/focus-order.html

@@ -124,17 +124,19 @@ <h2>Examples of Focus Order</h2>
             key expands a node, then using the down arrow key moves into the newly expanded nodes.
          </li>
 
-         <li>A web page implements modeless dialogs via scripting. When the trigger button is activated,
+         <li>A web page implements modeless dialogs. When the trigger button is activated,
             a dialog opens. The interactive elements in the dialog are inserted in the focus order
             immediately after the button. When the dialog is open, the focus order goes from the
             button to the elements of the dialog, then to the interactive element following the
             button. When the dialog is closed, the focus order goes from the button to the following
             element.
          </li>
 
-         <li>A web page implements modal dialogs via scripting. When the trigger button is activated,
+         <li>A web page implements modal dialogs. When the trigger button is activated,
             a dialog opens and focus is set within the dialog. As
-            long as the dialog is open, focus is limited to the elements of the dialog. When the
+            long as the dialog is open, all web page content outside the dialog becomes 
+            inert and cannot receive focus (though, depending on implementation, the focus cycle
+            might still include user agent controls). When the
             dialog is dismissed, focus returns to the button or the element following the button.
          </li>
 

understanding/20/no-keyboard-trap.html

@@ -64,9 +64,10 @@ <h2>Examples of No Keyboard Trap</h2>
                prior to the applet as well as within the applet itself.</dd>
          <dt>A modal dialog box</dt>
          <dd>A web application brings up a dialog box. At the bottom of the dialog are two buttons,
-               Cancel and OK. When the dialog has been opened, focus is trapped within the dialog;
-               tabbing from the last control in the dialog takes focus to the first control in the
-               dialog. The dialog is dismissed by activating the Cancel button or the OK button.</dd>
+               Cancel and OK. When the dialog has been opened, all web page content outside the dialog becomes 
+               inert and cannot receive focus (though, depending on implementation, the focus cycle
+               might still include user agent controls).
+               The dialog is dismissed by activating the Cancel button or the OK button.</dd>
       </dl>
 
    </section>