doc(std): clarify NonZero<T> usage limitation in doc comment

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

core/src/num/nonzero.rs

@@ -111,6 +111,15 @@ impl_zeroable_primitive!(
 /// ```
 ///
 /// [null pointer optimization]: crate::option#representation
+///
+/// # Note on generic usage
+///
+/// `NonZero<T>` can only be used with some standard library primitive types
+/// (such as `u8`, `i32`, and etc.). The type parameter `T` must implement the
+/// internal trait [`ZeroablePrimitive`], which is currently permanently unstable
+/// and cannot be implemented by users. Therefore, you cannot use `NonZero<T>`
+/// with your own types, nor can you implement traits for all `NonZero<T>`,
+/// only for concrete types.
 #[stable(feature = "generic_nonzero", since = "1.79.0")]
 #[repr(transparent)]
 #[rustc_nonnull_optimization_guaranteed]