[LangRef] Correct documentation for `roundeven` (#125452)
Langref for `roundeven` implies that the C standard function `roundeven`
may raise floating point exceptions. However, this is not correct; C23
does not mention exceptions for `roundeven`, and per [1] `FE_INEXACT` is
never raised.
Clarify that LLVM's `roundeven` behaves the same.
[1]: https://en.cppreference.com/w/c/numeric/math/roundeven
