man: we need to be more careful with the unit search paths we document

man: we need to be more careful with the unit search paths we document

We generally document the suggested paths, not the paths possible in
weird, non-standard setups. We do this in order to not confuse
administrators/users unnecessarily and to push people to install things
into the same directories on all distributions.

We are PID 1 after all, the really basic building block of the OS.
Unlike for an app there's very little benefit in being entirely
relocatable.