bug#36076: Manual should clarify that glibc-utf8-locales is needed by default on foreign distros

[Jack Hill]

Hi Guix,

While setting up Guix on a foreign distribution (CentOS 7), I elected to 
use the full glibc-locales while following section 2.6.1 of the manual for 
application setup. I installed the glibc-locales package in both my user's 
profile and root's so that the locales would be available to guix-daemon.

However, I was surprised that even though I had the full set of locales 
available guix-daemon couldn't find the locale it was looking for. This is 
because the default systemd unit for guix-daemon configures it to use the 
en_US.utf8 locale which is provided by glibc-utf8-locales (en_US.UTF-8 is 
in glibc-locales).

I think it would be good to clarify in the manual which locale package is 
needed when using the default daemon configuration on systemd foreign 
distros.

I also talked about my experience on help-guix: 
https://lists.gnu.org/archive/html/help-guix/2019-06/msg00024.html

Best,
Jack