Git’s manpages can be exported as an info manual called
gitman. Magit’s own info manual links to nodes in that manual instead of the actual manpages because Info doesn’t support linking to manpages.
Unfortunately some distributions do not install the
gitman manual by default and you will have to install a separate documentation package to get it.
Magit patches Info adding the ability to visit links to the
gitman Info manual by instead viewing the respective manpage. If you prefer that approach, then set the value of
magit-view-git-manual-method to one of the supported packages
(setq magit-view-git-manual-method 'man)