Fixes #11420: Update migration guide on split of .visible-/.hidden-desktop...

Fixes #11420: Update migration guide on split of .visible-/.hidden-desktop into .visible-/.hidden-md and .visible-/.hidden-lg

getting-started.html

@@ -379,7 +379,7 @@ bootstrap/
           </tr>
           <tr>
             <td><code>.visible-desktop</code></td>
-            <td><code>.visible-md</code></td>
+            <td>Split into <code>.visible-md .visible-lg</code></td>
           </tr>
           <tr>
             <td><code>.hidden-phone</code></td>
@@ -391,7 +391,7 @@ bootstrap/
           </tr>
           <tr>
             <td><code>.hidden-desktop</code></td>
-            <td><code>.hidden-md</code></td>
+            <td>Split into <code>.hidden-md .hidden-lg</code></td>
           </tr>
           <tr>
             <td><code>.input-small</code></td>