[globalisel][legalizer] Attempt to write down the minimal legalization rules

Summary:
There aren't very many requirements on the legalization rules but we should
document them.

Reviewers: aditya_nandakumar, volkan, bogner, paquette, aemerson, rovka, arsenm, Petar.Avramovic

Subscribers: wdng, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62423

# Conflicts:
#	llvm/docs/GlobalISel.rst

llvm-svn: 368321