Fix failures with Debian's APT repository onion service

Once in a while, sometimes a bit more like this month, we receive reports about failures with the apt repository onion service:

Err :1 tor+http://vwakviie2ienjx6t.onion/debian buster/main amd64 [some application] all Connexion à 127.0.0.1: 9050 (127.0.0.1) impossible. - connect (111: Connection refused)

Could we add a note to known issues about this?

We say to the users:

• This is usually a temporary issue, maybe the server was under maintenance
• Please remember to do apt update before doing apt install