I see plenty of advice on technical writing, which typically concentrates on how to structure things on a large scale, and how to make the words clear on a small scale.

But when (as very often happens) I'm reading documentation and I find it's less than satisfactory, the main problem is rarely the structure or the wording.

The main problem is nearly always that the information I need just isn't there.

So please, if you're training people to write, tell them to always keep these in mind as priorities: "Are the terms I used here defined elsewhere in the document, or else ones that I can assume all readers already know?", and "Have I actually said what this thing does?".