Add link to date format to avoid confusion

Fixes #697
The Italian and French translations are incorrect as the translators changed the date form from "yyyy" and "dd" to the first letter of the words in those languages. I'm adding a link to the official documentation for the date format in PHP and Javascript respectively for each of these to help translation.

includes/admin/class-wp-job-manager-admin.php

@@ -45,7 +45,7 @@ public function admin_enqueue_scripts() {
 			wp_enqueue_script( 'job_manager_admin_js', JOB_MANAGER_PLUGIN_URL. '/assets/js/admin.min.js', array( 'jquery', 'jquery-tiptip', 'jquery-ui-datepicker' ), JOB_MANAGER_VERSION, true );
 
 			wp_localize_script( 'job_manager_admin_js', 'job_manager_admin', array(
-				'date_format' => _x( 'yy-mm-dd', 'Date format for jQuery datepicker', 'wp-job-manager' )
+				'date_format' => _x( 'yy-mm-dd', 'Date format for jQuery datepicker. See http://api.jqueryui.com/datepicker/#utility-formatDate', 'wp-job-manager' )
 			) );
 		}
 

includes/admin/class-wp-job-manager-writepanels.php

@@ -84,7 +84,7 @@ public function job_listing_fields() {
 				'label'       => __( 'Listing Expiry Date', 'wp-job-manager' ),
 				'priority'    => 11,
 				'classes'     => array( 'job-manager-datepicker' ),
-				'placeholder' => _x( 'yyyy-mm-dd', 'Date format placeholder', 'wp-job-manager' ),
+				'placeholder' => _x( 'yyyy-mm-dd', 'Date format placeholder. http://php.net/manual/en/function.date.php', 'wp-job-manager' ),
 				'value'       => metadata_exists( 'post', $post->ID, '_job_expires' ) ? get_post_meta( $post->ID, '_job_expires', true ) : calculate_job_expiry( $post->ID ),
 			);
 		}