[SYCL][Doc] Specify negative conversion to E8M0 (#22303)
We decided that the result of converting a negative value to E8M0 should
be unspecified. This never happens in practice, so we don't expect to
see applications that really do this. The hardware instruction specs we
looked at don't specify the behavior in this situation. Therefore, if we
want SYCL to implement this in hardware, we have to say that the result
is unspecified.
