TableGen: some LangRef doc fixes

Summary: Change-Id: I1442e2daa09cab727a01d8c31893b50e644a5cd3

Reviewers: tra, simon_tatham, craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47530

Change-Id: I397655dd18b7ff978c1affa3174740d9c1a82594
llvm-svn: 333901