Doc bug: Postgres account not available

[Pierre Neidhardt <mail@ambrevar.xyz>]

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Hi!

Apparently our Postgres service changed a little bit recently.  While it
was easy to setup in the past, this week it cost me a few hours, trying
to figure out how to do a simple thing like

--8<---------------cut here---------------start------------->8---
$ psql -U postgres
psql: FATAL:  Peer authentication failed for user "postgres"
--8<---------------cut here---------------end--------------->8---

Eventually, I discovered https://issues.guix.info/38198.
Turns out the authentication policy is now "peer" and the shell-less
'postgres' user cannot be used without a shell.

As such, to administrate PostgresQL we need first to switch to the
postgres user as follows:

--8<---------------cut here---------------start------------->8---
  sudo su postgres -s psql
--8<---------------cut here---------------end--------------->8---

This was not a very obvious find on the Internet.
Can we make this easier?  The above thread suggests to leave the setup
as is and to document it.

Thoughts?  Gábor, Giovanni?

-- 
Pierre Neidhardt
https://ambrevar.xyz/

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