TL;DR: documentation should be treated, i.e. planned, estimated and maintained, as any other functional part of a project.
I don’t always write documentation but when I do, I believe it’s more important it to be up-to-date than to exist. That it, it’s better to do not have any documentation than to have an out-of-date one. Because outdated documentation is misleading, misinforms your clients, might do more harm than good.