document the new product variant id template variable

lib/WebGUI/Help/Asset_Product.pm

@@ -48,6 +48,7 @@ our $HELP = {
             { 'name' => 'addvariant_label' },
             {   'name'      => 'variant_loop',
                 'variables' => [
+                    { 'name' => 'variant_id' },
                     { 'name' => 'variant_controls' },
                     { 'name' => 'variant_sku' },
                     { 'name' => 'variant_title' },

lib/WebGUI/i18n/English/Asset_Product.pm

@@ -723,6 +723,11 @@ be useful, others may not.|,
 		lastUpdated => 1210029452,
 	},
 
+	'variant_id' => {
+		message => q|The unique identifier for this variant.|,
+		lastUpdated => 1221777106,
+	},
+
 	'variant_controls' => {
 		message => q|An icon toolbar for editing, deleting, or reordering this variant.|,
 		lastUpdated => 1210029452,