document PartialOrd difference to simd_min and simd_max

Avoid migration pitfall from packed_simd as described in #402.

crates/core_simd/src/vector.rs

@@ -925,6 +925,7 @@ where
     }
 }
 
+/// Lexicographic order. For the SIMD elementwise minimum and maximum, use simd_min and simd_max instead.
 impl<T, const N: usize> PartialOrd for Simd<T, N>
 where
     LaneCount<N>: SupportedLaneCount,