Make Link Context Menu Optional

Made the following changes Per discussion of issue #228 in Feb 6 authoring practices meeting.

modified aria-practices.html:
In the link pattern section, keyboard subsection, marked the context menu key (shift+f10) as optional.

modified examples/link/link.html:
In the example introduction, added the following sentence to the note encouraging authors to use native HTML links:
"Browsers provide valuable functionality for native HTML links, e.g., open the target in a new window and copy the target URL to the system clipboard."

aria-practices.html

@@ -1092,7 +1092,7 @@ <h3>Link</h3>
         <h4>Keyboard Interaction</h4>
         <ul>
           <li><kbd>Enter</kbd>: Executes the link and moves focus to the link target.</li>
-          <li><kbd>Shift + F10</kbd>: Opens the link's context menu.</li>
+          <li><kbd>Shift + F10</kbd> (Optional): Opens a context menu for the link.</li>
         </ul>
       </section>
       <section class="notoc">

examples/link/link.html

@@ -26,7 +26,10 @@ <h1>Link Examples</h1>
     <a href="../../#link">design pattern for link</a>.
      The link pattern is used when it is necessary for elements other than the HTML <code>a</code> element to have link behaviors.
   </p>
-  <p><strong>Note:</strong> Use the HTML <code>a</code> element to create links whenever possible.</p>
+  <p>
+    <strong>Note:</strong> Use the HTML <code>a</code> element to create links whenever possible.
+    Browsers provide valuable functionality for native HTML links, e.g., open the target in a new window and copy the target URL to the system clipboard.
+  </p>
   <section>
     <h2 id="ex_label">Examples</h2>
     <div role="separator" id="ex_start_sep" aria-labelledby="ex_start_sep ex_label" aria-label="Start of"></div>