Right now it is not obvious that
menu_name is only globally available and it needs to get passed into the
menu.html.twig to render e.g. in the toplevel
This is probably only necessary for themers / developers who hit the first time a twig macro (was it for me) but
menu.html.twig might be one of the first templates with which a themer / developer gets in contact and would ease the usability of the template.
Mention in the template header / comments that
menu_name needs to get past into the
menu_links macro because it is only globally available like