[LangRef] fix formatting in FP descriptions; NFC
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@326913 91177308-0d34-0410-b5e6-96231b3b80d8

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