I would argue that it's a back-and-forth anyway. Especially with more technical stuff, I write a kind-of outline first, (actually bullet points with sub-points, where each bullet point is a short sentence). That way, I have short, simple sentences that I can move around until it "fits", then turn everything into text.
But in any case, the important thing is "figuring out what to say". And that is a process, not a single step. So "writing a first draft, then iterating until it fits" is IMHO the "normal" way to do it anyway.