[LangRef] fix description and examples of fptrunc

As noted in PR36966:
https://bugs.llvm.org/show_bug.cgi?id=36966

The old description doesn't match what we do in code, 
so this just fixes the documentation to avoid confusion.

Differential Revision: https://reviews.llvm.org/D45190


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329065 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed