jml started a good series with some documentation themes earlier today; I figured I'd contribute another few ideas. The most immediate one comes up as a further point to the reposted card: your documentation should be based on where the other person begins
To clarify that a bit: find the crossover between implicit and necessary knowledge. "Setting up a vhost", for instance, presumes knowing what a vhost is
(this can be catered to the so-called target market consumers of the thing you're documenting)