Update the inline docs for `add_shortcode()` to eliminate suggestion to use `extract()`.

See #22400.


git-svn-id: https://develop.svn.wordpress.org/trunk@28413 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/shortcodes.php

@@ -65,12 +65,12 @@ $shortcode_tags = array();
  * <code>
  * // [bartag foo="bar"]
  * function bartag_func($atts) {
- * 	extract(shortcode_atts(array(
+ * 	$args = shortcode_atts(array(
  * 		'foo' => 'no foo',
  * 		'baz' => 'default baz',
- * 	), $atts));
+ * 	), $atts);
  *
- * 	return "foo = {$foo}";
+ * 	return "foo = {$args['foo']}";
  * }
  * add_shortcode('bartag', 'bartag_func');
  * </code>