Edit: I misread the original post; the below is referring to /doc pages, not the wiki.musicbrainz.org site that’s the source of those pages. Sorry for the confusion.
I posted about this a while back at Making documentation pages usable on mobile devices.
The stylesheet for documentation pages looks like it’s at https://github.com/metabrainz/musicbrainz-server/blob/master/root/static/styles/extra/wikidocs.less, so I’ve uploaded my proposed fix at https://github.com/metabrainz/musicbrainz-server/pull/3187.