When writing documentation, you should not only think about the information you want to convey: you also need to think about the language, structure, and presentation of your copy.
The writing process is generally iterative as you build your text from an outline to real bits of text and eventually the full copy of your article. When writing for the web, you can also add navigation through links and headings.
A good article which provides some quick tips on good technical writing. The main idea is to plan it well, structure it well (in terms of content and typesetting), make it easy to understand, don’t be condescending and don’t lie. There are some tips for web technical writing as well.