The short answer is this: you can write anything you want or need to about any project, Scrum or otherwise. Scrum doesn't tell you how to document, but it doesn't tell you not to. The way you document is in itself irrelevant to Scrum.
That said, if you need to understand how the software currently works, a document will not help you. Documentation often lies. If you're trying to understand how the system works, a document will only tell you what people think or want to believe is the truth.
What you should consider, is to use executable specifications and Test Driven Development to prove that what you believe the software does is actually true. automated tests combine documentation, examples and regression tests all into one offer.