[ThinLTO] Update LangRef doc for summary parsing

Summary:
Remove note about summary being ignored. Update to reflect the
fact that summary is now parsed by llvm-as.

While here, fix one summary format that changed since the initial
implementation.

Reviewers: dexonsmith

Subscribers: inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D51540

llvm-svn: 342479