[LangRef] clarify semantics of the frem instruction

As noted in D40594, the frem instruction corresponds to fmod() except that it can't set errno.
I modified the text that we currently use for intrinsics that map to libm functions and applied
it to frem.

Differential Revision: https://reviews.llvm.org/D40629



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319437 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed