Fix mistakes in parameter documentation and add some missing param docs. See #19756.

git-svn-id: http://svn.automattic.com/wordpress/trunk@19702 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/screen.php

@@ -565,7 +565,8 @@ final class WP_Screen {
 	 *
 	 * @since 3.3.0
 	 *
-	 * @param string
+	 * @param string $option Option ID.
+	 * @param mixed $key Optional. Specific array key for when the option is an array.
 	 */
 	public function get_option( $option, $key = false ) {
 		if ( ! isset( $this->_options[ $option ] ) )