nixos/manual/manpages: add description on previewing manpage files
I was a bit lost with the new manpage format and it took me some time to
find the corresponding pull request by @pennae with a very helpful
comment of @alyssais that mentioned this:
https://github.com/NixOS/nixpkgs/pull/213256#issuecomment-1407713215
As @pennae noted, the file path is only a fallback if it cannot be resolved in the man database
Co-authored-by: pennae <82953136+pennae@users.noreply.github.com>
516a9816a3