The real shell functions are not documented by separate man pages, but with the help help built-in. You will have to override this. But even I would not look for information.
Just create normal man pages and throw them in /usr/local/man/manX or /usr/local/share/man/manX - regardless of what your distribution already provides. Check /etc/manpath.config that this directory is already mentioned there. Thus, no one should bother in their startup files with the MANPATH environment MANPATH .
Each man page should also contain a clearly visible section explaining that this is a function, not a command, and what is the difference.
After that, the social part begins: talk about this documentation every time. For everyone, I mean everyone, not only suitable .:-)
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