Document systemd dependencies on network-online.target

[gollux]

Hit this problem at the IOI and the solution is more tricky than expected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.04%. Comparing base (dfa0b66) to head (a8e7b56).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1709   +/-   ##
=======================================
  Coverage   55.04%   55.04%           
=======================================
  Files         339      339           
  Lines       27401    27401           
=======================================
+ Hits        15083    15084    +1     
+ Misses      12318    12317    -1     

Flag	Coverage Δ
functionaltests	0.00% <ø> (ø)
unittests	55.04% <ø> (+<0.01%)	⬆️

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