For me, at least, that's true. But then, I've written a lot of words in my life.
Now, I will plan the documentation away from music, and I'll edit it away from a computer, but music gives you a "permission to suck" and just get words on the paper. Once you have words on the paper, you have something that can be molded.
I'm sorry, documentation requires less concentration?
I find (I hypothesize) that because I'm doing something which balances the hemispheres more (and requires me to get inside the head of my user) music doesn't work for documentation, it's too distracting. Programming yes, ambient works well, but writing...