[mlir] Expand shape functions in ShapeInference doc

Summary:
Start filling in some requirements for the shape function descriptions
that will be used to derive shape computations. This requiement part may
later be reworked to be part of the "context" section of shape dialect. Without
examples this may be a bit too abstract but I hope not (given mappings to
existing shape functions).

Differential Revision: https://reviews.llvm.org/D73572