Unfortunately this is a bit of a mess but you need to understand the history and politics here. First off, Debian, as well as the other Unix and Linux distributions are a collection of lots of different things from differnet places and you get an operating system out of it all. Something like Microsoft's Windows or the old mainframe operating systems like DEC's VMS or the various IBM operating systems, these operating systems were put out by a company which had complete control over all aspects of the system and it's documentation. GNU/Linux meaning Debian, Ubuntu, RedHat, or any of the hundred or so linux distrubtions just are not this way. Unix and Linux distrubtions are a collection of many things from many places. The 'ls' man page you mention is part of the GNU utilities, not written by the Debian project, nor is the shell like sh, csh, bash, zsh etc.. Unix Man pages have been around for many decades and each component usually (not always!) comes with a man page. There's no single company or organization that has any overarching responsility to make sure any individual man page is consistent with another. Furthermore, some things like some of the Gnu tools have documentation in a system called 'info' and there's often files distributed in /usr/share/docs and then some projects document things in web pages and in markdown files like readmes in git repositories. There just isn't a single point of documentation and I doubt you'll get everyone to double-document things by making man pages AND writing documentation in some global documentation repository. It is unfortunate that today, sometimes the best documentation is by doing a web search and reading though things on sites like stackexchange or perople's personal blogs. I say unfortunate but it works. Don't get me wrong, it would be great if there was like a wikipedia for all this but I doubt it will ever happen, and Debian is just one of many different projects that consumes as well as produces things.
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