Writing documentation is painful, because it reveals inconsistencies and fuzzy areas in your design. If you're not paying attention, you may associate that pain with the act of writing documentation. In reality, the problem lies with your designs.
I regularly run into the very issue you describe. I then update my designs.
To me, documentation is also tool to verify my designs.
If you're designing, say, a RESTful API, it's a good idea to write a couple of client programs in different languages. See what kind of hell you would be putting your users through.
The idea generalizes, as well. Marketing is, sort of, documentation for the value proposition of your business. If you can't make the marketing work, maybe your business fundamentals need to be adjusted instead of hiring a “better marketer”.