Fix inaccurate `std::intrinsics::simd` documentation This addresses two issues: - the docs on comparison operators (`simd_gt` etc.) said they only work for floating-point vectors, but they work for integer vectors too. - the docs on various functions that use a mask did not document that the mask must be a signed integer vector. Unsigned integer vectors would cause invalid behavior when the mask vector is widened (unsigned integers would use zero extension, producing incorrect results). r? ``@workingjubilee``