wp-content/themes/twentyfifteen/functions.php:574Display descriptions in main navigation.
$item_outputstring$itemWP_Post$depthint$argsstdClassstringfunction twentyfifteen_nav_description( $item_output, $item, $depth, $args ) { if ( 'primary' === $args->theme_location && $item->description ) { $item_output = str_replace( $args->link_after . '</a>', '<div class="menu-item-description">' . $item->description . '</div>' . $args->link_after . '</a>', $item_output ); } return $item_output;}Introduced in Twenty Fifteen 1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-content/themes/twentyfifteen/functions.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.