Dialog Pattern: Address @jnurthen review feedback in issue #325

Modify the modal dialog design pattern section in aria-practices.html to:

1. Recommend tabindex of -1 when focusing static elements at the top of large dialogs.
2. Rephrase initial focus guidance for potentially destructive actions.
3. Add aria-modal to states and properties.

aria-practices.html

@@ -530,15 +530,16 @@ <h4>Keyboard Interaction</h4>
               <li>In all circumstances, focus moves to an element contained in the dialog.</li>
               <li>Unless a condition where doing otherwise is advisable, focus is initially set on the first focusable element.</li>
               <li>
-                If the content is large enough that focusing the first interactive element could cause the beginning of content to scroll out of view, 
-                it is advisable to add <code>tabindex=<q>0</q></code> to a static element at the top of the dialog, such as the dialog title or first paragraph, and initially focus that element.
+                If content is large enough that focusing the first interactive element could cause the beginning of content to scroll out of view, 
+                it is advisable to add <code>tabindex=<q>-1</q></code> to a static element at the top of the dialog, such as the dialog title or first paragraph, and initially focus that element.
               </li>
               <li>
-                If the dialog is asking the user to confirm a destructive action, it may be advisable to set focus on the least destructive action, especially if undoing the action is difficult or impossible.
+                If a dialog contains the final step in a process that is not easily reversible, such as deleting data or completing a financial transaction, 
+                it may be advisable to set focus on the least destructive action, especially if undoing the action is difficult or impossible.
                 The <a href="#alertdialog">Alert Dialog Pattern</a> is often employed in such circumstances.
               </li>
               <li>
-                If the dialog is limited to interactions that either provide additional information or continue processing,
+                If a dialog is limited to interactions that either provide additional information or continue processing,
                 it might set focus to the element deemed to be most frequently desired, such as a <q>OK</q> or <q>Continue</q> button.
               </li>
             </ul>
@@ -554,6 +555,7 @@ <h4>Keyboard Interaction</h4>
         <h4>WAI-ARIA Roles, States, and Properties</h4>
         <ul>
           <li>The element that serves as the dialog container has a role of <a href="#dialog" class="role-reference">dialog</a>.</li>
+          <li>The dialog container element has <a href="#aria-modal" class="property-reference">aria-modal</a> set to <code>true</code>.</li>
           <li>The dialog has either:
             <ul>
               <li>The <a href="#aria-labelledby" class="property-reference">aria-labelledby</a> property set to refer to the visible dialog title.</li>