bug#52862: Clarify note about sudo in 3.7 After System Installation

[Akira Kyle]

Hi,

I'm trying to get going with guixsd for the first time and I just
spent awhile trying to figure out what was going wrong trying to run
'guix pull' and 'guix system reconfigure /etc/config.scm'.

The issue was that I was running both in shell obtained via 'sudo su'
from my user shell. I had read the note about this in section "3.7
After System Installation" and thought since, following the note, I
was running both in a root shell, this would be fine. Coming from nix,
this is fine, however I see now that the environment and path are
incorrect in a 'sudo su' shell as the guix that was being used was the
system guix from /run/current-system/profile and not
/root/.config/guix/current which led to the effective system downgrade
that running guix system reconfigure kept warning about.

I would suggest updating the documentation to explicitly say 'sudo su'
should not be used since I expect there are many others like me who
are ignorant to the differences between 'sudo su' and 'sudo -i'

Thanks,
Akira