Spinbutton Pattern: Clarify Home and End key guidance

For issue #446, Modified the spinbutton pattern in aria-practices.html.
Added language to make it clear that
Home and End are only needed for spinbuttons that have min and max values.

aria-practices.html

@@ -1819,8 +1819,8 @@ <h4>Keyboard Interaction</h4>
           </li>
           <li><kbd>Up Arrow</kbd>: Increases the value.</li>
           <li><kbd>Down Arrow</kbd>: Decreases the value.</li>
-          <li><kbd>Home</kbd>: Sets the spinbutton to the minimum value.</li>
-          <li><kbd>End</kbd>: Sets the spinbutton to the maximum value.</li>
+          <li><kbd>Home</kbd>: If the spinbutton has a minimum value, sets the value to its minimum.</li>
+          <li><kbd>End</kbd>: If the spingutton has a maximum value, sets the value to its maximum.</li>
           <li><kbd>Page Up</kbd> (Optional): Increases the value by a larger step than <kbd>Up Arrow</kbd>.</li>
           <li><kbd>Page Down</kbd> (Optional): Decreases the value by a larger step than <kbd>Down Arrow</kbd>.</li>
         </ul>