As others have said... if you have extra time to optimize how you write documentation, fine, but just get something written. Actually... if you have extra time,

As others have said... if you have extra time to optimize how you write documentation, fine, but just get something written. Actually... if you have extra time, take the time to write your documentation well - make it clear, consider who the audience is, spend some time thinking about how to organize the information, and so on. If you still have extra time, then it's time to worry about latex vs. xml vs. html vs. plain text vs. Word vs. Wiki vs. RTF. The company I work for has a wiki that most teams use for documentation, and that generally works pretty well - for most uses, you can use the WYSIWYG editor, and if you need something more precise you can use the wiki text editor.