I was working as a freelancer for a long time and I had a quite simple solution for lack of documentation. Before the first meeting I'd send a customer 12-page form to fill which covered most of the "what and why" of the project. It quickly turned out that most of people have no idea whatsoever about the product they wanted to buy. I know that it's partly our job to guide people through rough path, but if all customer knows is "I want a website" it's better to quit early.

Sometimes I waited for a filled form for few months or even more (not paying much attention), and some of these projects were very successful. If I'd have started building right away, I'd spent these few months struggling and waiting for decisions to be made. What is even worse, I'd probably estimate my time and salary based on wrong assumptions, so I would be mad and underpaid. The project would take way longer than I estimated, so customer would be angry too.

Do yourself a favor and never start coding before final documentation arrives.