Add missing phpdoc block for `export_date_options()`.

Props siobhan for the initial patch. See #20425


git-svn-id: https://develop.svn.wordpress.org/trunk@25490 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/export.php

@@ -109,6 +109,16 @@ if ( isset( $_GET['download'] ) ) {
 
 require_once ('admin-header.php');
 
+/**
+ * Create the date options fields for exporting a given post type.
+ *
+ * @global wpdb      $wpdb      WordPress database object.
+ * @global WP_Locale $wp_locale Date and Time Locale object.
+ *
+ * @since 3.1.0
+ *
+ * @param string $post_type The post type. Default 'post'.
+ */
 function export_date_options( $post_type = 'post' ) {
 	global $wpdb, $wp_locale;