[WebAssembly][NFC] Document stackifier tablegen backend

Summary:
Add comments to help readers avoid having to read tablegen backends to
understand the code. Also remove unecessary breaks from the output.

Reviewers: dschuff, aheejin

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D51371

llvm-svn: 340864