Docs: Describe Derivation of Constraints; Don't Present Them

[aszepieniec]

Currently, the AIR constraints documentation is difficult to read and even more difficult to maintain. The constraints, as presented, are not informative. We should not be presenting the them but rather describing where they came from and how to (re-)derive them.

Note that someone who is unfamiliar with our codebase might be a little confused about how a description maps onto a constraint. So we need an additional section explaining how to derive a concrete set of constraints from a textual description of what they are supposed to do.