Should we really point to git as an example of great documentation? I mean, this exists and has to have a giant banner at the top saying "these are NOT REAL": https://git-man-page-generator.lokaltog.net/
GNU historically never made man pages: they used Texinfo for documentation, possibly combined with a tool to convert Texinfo to man pages. That shows in the result: an entire manual all joined together in a single unreadably long manual page. The bash man page is a nice example of that. I'm not sure if the use of Texinfo is still a requirement for GNU projects.
The verbosity of the generated man pages for GNU software probably did influence a lot of non-GNU Linux software.
Many GNU manpages are generated with help2man, which causes the opposite problem, i.e. they are too terse. (Also, their typographical quality is often low.)