Any docs for wp_nav_menu’s “items_wrap” argument?

The parameter 'items_wrap' for wp_nav_menu() defaults to:

'<ul id="%1$s" class="%2$s">%3$s</ul>' 

This a a template that is parsed with sprintf():

$nav_menu .= sprintf( 
    $args->items_wrap
,   esc_attr( $wrap_id )    // %1$s
,   esc_attr( $wrap_class ) // %2$s
,   $items                  // %3$s
);

The numbered placeholders – %1$s, %2$s, %3$s – refer to the arguments after the first argument in sprintf(). The percent sign marks a placeholder, the number the position and the type s means it should be treated as a string.
Do not change the type unless you really know what you do. 🙂

• $wrap_id is the parameter 'menu_id' if you have it set, else it is 'menu-' . $menu->slug.
• $wrap_class is the parameter 'menu_class' if you have it set, else it is empty.
• $items is a string of the inner content of the menu.

Let’s say you don’t need a class. Just omit the second string:

wp_nav_menu( array( 'items_wrap' => '<ul id="%1$s">%3$s</ul>' ) );

If you don’t need the class and the id, and you want another container (because you used a custom walker):

wp_nav_menu( array( 'items_wrap' => '<div>%3$s</div>' ) );

The main point is: You have to use the numbers for the replacements given in wp_nav_menu(). %3$s is always the list of items.