docs(linalg): fill in the empty Safety section on load_unaligned

rust/lance-linalg/src/simd.rs

@@ -58,6 +58,20 @@ pub trait SIMD<T: Num + Copy, const N: usize>:
     /// Load unaligned data from memory.
     ///
     /// # Safety
+    ///
+    /// `ptr` must be valid for reads of `N` elements of `T`.
+    ///
+    /// Most implementations also require a target feature that the build's
+    /// baseline may not provide, and none of them check for it at runtime:
+    ///
+    /// - `f32x8`, `f32x16`, `f64x4`, `f64x8` and `i32x8` need AVX on x86_64 and
+    ///   LASX on loongarch64;
+    /// - `u8x16` needs nothing beyond the baseline on any target.
+    ///
+    /// Calling one without its feature is undefined behaviour rather than a
+    /// wrong answer, so either build with the feature enabled, as
+    /// `.cargo/config.toml` does for `x86_64-unknown-linux-gnu`, or check with
+    /// `is_x86_feature_detected!` first. See #8872.
     unsafe fn load_unaligned(ptr: *const T) -> Self;
 
     /// Store the values to aligned memory.