Update systemd documentation and systemd examples

[tomodachi]

Hi people

It's not clear from the systemd example exactly how a user systemd service works.

With the given example spotifyd will only be started when the user logs in to the system and it will also be stopped as soon as the user logs out.

That might not be a expected behaviour for a setup like a small headless device always connected to your speakers. So I've added an example for that and made some clarifications.

[stale[bot]]

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[slondr]

@robinvd this look good to you?