Add footnote

Author: calebzulawski

crates/core_simd/src/core_simd_docs.md

@@ -30,8 +30,10 @@ Instead, they map to a reasonable implementation of the operation for the target
 
 Consistency between targets is not compromised to use faster or fewer instructions.
 In some cases, `std::arch` will provide a faster function that has slightly different behavior than the `std::simd` equivalent.
-For example, [`_mm_min_ps`](`core::arch::x86_64::_mm_min_ps`) can be slightly faster than [`SimdFloat::simd_min`], but does not conform to the IEEE standard also used by [`f32::min`].
+For example, [`_mm_min_ps`](`core::arch::x86_64::_mm_min_ps`)[^1] can be slightly faster than [`SimdFloat::simd_min`], but does not conform to the IEEE standard also used by [`f32::min`].
 When necessary, [`Simd<T, N>`] can be converted to the types provided by `std::arch` to make use of target-specific functions.
 
 Many targets simply don't have SIMD, or don't support SIMD for a particular element type.
 In those cases, regular scalar operations are generated instead.
+
+[^1]: `_mm_min_ps(x, y)` is equivalent to `x.simd_lt(y).select(x, y)`