I don't buy that argument. People are using the software now. And software is never finished. There is always more to be done. So if you wait until it is done, then you never document it.
My personal experience, and very strongly felt, is that writing documentation is a key part of finalising and debugging specification. So many times have I experienced this. Only when you write the documentation do you realise some of the issues that users will face. Deal with them before releasing and you don't have to spend as much time in back compat hell.
My view is that writing documentation in a timely fashion actually saves time and resources in the long run.