Agile requires the correct level of documentation, not too much or little, no "good luck" statements (I can't remember the SO question that used this.)
So if you are writing User stories and the like, how do you start out in the initial negotiations? You don't just write a contract or statement or work that says, "Build a Shopping Cart Website" (my thought is no).
It look like Agile does not really like a Software Design Document, at least not right off, but isn't that what you need from a contractual viewpoint? I realize things change for requirements in the actual building of the software (people forget things, we need this, etc.), but I am concerned about the initial signing.
Edit: https://www.joelonsoftware.com/2000/10/03/painless-functional-specifications-part-2-whats-a-spec/ is old but still good I think. How does this fit with Agile?