Systemd service and socket authentication

[devBrouwer]

I addded some documentation for configuration that might be necessary to the systemd service file when using socket authentication. For example on Arch Linux, I needed to do these changes.

[Arvedui]

I'm not entirely convinced that this belongs here, as it is quite system specific.
But alright, I will merge it, please squash your commits first though.

[AlphaJack]

Thank you, for me it was enough running sudo systemctl edit --full radicale and then removing "AF_UNIX" from "RestrictAddressFamilies="

[ewtoombs]

I did something similar, but I used sudo systemctl edit radicale and added the lines

[Service]
RestrictAddressFamilies=
RestrictAddressFamilies=~AF_PACKET AF_NETLINK

When you do it this way, it's more maintainable.