Explain the difference between 'typical role' metadata and `user_role`

[plocket]

https://github.com/SuffolkLITLab/docassemble-AssemblyLine-documentation/pull/122#discussion_r645775371

Make sure of this explanation:

I think user_role is the runtime variable, and typical role is the "compile time" value. If typical role anything but 'unknown', then user_role is just set to that, and if it is unknown, then we ask for the user_role. I only assume we set it in the weaver because it's clearer what's happening, without a bunch of DA magic happening behind the scenes, and maybe for backwards compatibility?