Getting good test documentation is a consistent challenge. Agile proposes that you should go very light on documentation, and while test documentation does not need to be heavy, it does need to be clear and cover all that the product is intended to do so you can ensure testing is consistent and results are recorded. Here's how to overcome some major barriers to getting good test documentation.
One of the lines in the Agile Manifesto is "Working software over comprehensive documentation." This doesn't mean that no documentation is produced, but instead that only documentation that brings value to the team and the customer should be created.