Landmark Practice: Add note to prevent dialog content wrapped in a landmark (pull #2818)

Resolve issue #2512 by adding the following note to principles of landmark design:

>Note that wrapping the content of a modal dialog in a landmark region is unnecessary. A landmark that wraps modal content cannot provide any benefit to users because it is not perceivable unless the modal is open. In addition, when a modal is open, a landmark containing its content is superfluous because the modal itself is a container that provides both a name and boundaries.

Author: mcking65

content/patterns/landmarks/examples/general-principles.html

@@ -80,6 +80,11 @@ <h1>General Principles</h1>
               <li>
                 Landmark roles can be nested to identify parent/child relationships of the information being presented.
               </li>
+              <li>
+                Note that wrapping  the content  of a modal dialog in a landmark region is unnecessary.
+                A landmark that wraps modal content cannot provide any benefit to users because it is not perceivable unless the modal is open.
+                In addition, when a modal is open, a landmark containing its content is   superfluous because the modal itself is a container that provides both a name and boundaries.
+              </li>
             </ul>
 
             <p style="font-weight: bold">Step 3: Label each area</p>

content/practices/landmark-regions/landmark-regions-practice.html

@@ -103,10 +103,13 @@ <h2>General Principles of Landmark Design</h2>
 
         <ul>
           <li>Assign landmark roles based on the type of content in the area.</li>
-
           <li><code>banner</code>, <code>main</code>, <code>complementary</code> and <code>contentinfo</code> landmarks should be top level landmarks.</li>
-
           <li>Landmark roles can be nested to identify parent/child relationships of the information being presented.</li>
+          <li>
+            Note that wrapping  the content  of a modal dialog in a landmark region is unnecessary.
+            A landmark that wraps modal content cannot provide any benefit to users because it is not perceivable unless the modal is open.
+            In addition, when a modal is open, a landmark containing its content is superfluous because the modal itself is a container that provides both a name and boundaries.
+          </li>
         </ul>
 
         <p id="aria_lh_step3"><strong>Step 3: Label areas</strong></p>