I believe it’s important to have detailed systems documentation, I wrote about it here -> the importance of good Systems Documentation.
Open source systems tend to be lacking in documentation because they are fast moving, rapidly changing projects and everyone wants to be involved in the glamourous stuff, not stuck writing systems documentation. I stumbled across this Tienda User Guide which Tessa Kriesel has shared.
This can give users a good overview of the Tienda system, although I would normally provide my clients with a bespoke document suited to their system. This is especially important if it’s been customised and looks or acts differently to a standard implentation. Tessa’s document is an excellent guide though.