This is a packaging thing, and I am not sure about this one. Sure Nginx is popular. But what about Hiawatha, Lighttp, $x, $y and $z? It would then make more sense to not specify any web server at all (or maybe as separate package). This would leave the choice to the admin.
Then, here is my call to all nginx experts to improve the documentation. It resides in a Github repo as well, and for sure will in Nextcloud, too. It’s just text after all. Bugs can be reported against specific problems (content and structure wise) and the nginx related documentation pages can be re-organized and enhanced.