rust: Add missing SAFETY documentation for ARef example

SAFETY comment in rustdoc example was just 'TODO'. Fixed.

Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>

Author: Oliver Mangold

rust/kernel/types.rs

@@ -498,15 +498,17 @@ impl<T: RefCounted> ARef<T> {
     ///
     /// struct Empty {}
     ///
-    /// # // SAFETY: TODO.
+    /// // SAFETY: The `RefCounted` implementation for `Empty` does not count references
+    /// // and never frees the underlying object. Thus we can act as having a
+    /// // refcount on the object that we pass to the newly created `ARef`.
     /// unsafe impl RefCounted for Empty {
     ///     fn inc_ref(&self) {}
     ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
     /// }
     ///
     /// let mut data = Empty {};
     /// let ptr = NonNull::<Empty>::new(&mut data).unwrap();
-    /// # // SAFETY: TODO.
+    /// // SAFETY: We keep `data` around longer than the `ARef`.
     /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
     /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
     ///