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 man or woman, e.g.:
(setq magit-view-git-manual-method 'man)