Document properties of internal type operations

[JukkaL]

Document basic properties of all type operations used within mypy, including compatibility, proper subtyping, joins and meets. The properties include things like associativity, symmetry, how they work with Any -- and there are many other properties potentially worth documenting. The documentation would be in the form of comments or docstrings within the source code.

[gvanrossum]

This just got moved down on the roadmap and I think it's not really a high priority (especially not for the core team).

[JukkaL]

We can revisit this for example when we have new core team members who would find this helpful.