[LangRef] fix formatting in FP descriptions; NFC

This is a clean-up step to reduce diffs ahead of real
changes to the FP semantics as discussed on llvm-dev:
http://lists.llvm.org/pipermail/llvm-dev/2018-February/121444.html



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326913 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed