rust: atomic: Update a safety comment in impl of `fetch_add()`

The safety comment used in the implementation of `fetch_add()` could be
read as just saying something it is true without justifying it. Update
the safety comment to include justification.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Link: https://patch.msgid.link/20260220-atomic-sub-v3-3-e63cbed1d2aa@kernel.org

rust/kernel/sync/atomic/internal.rs

diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
index b762dbd..ad810c2 100644
--- a/rust/kernel/sync/atomic/internal.rs
+++ b/rust/kernel/sync/atomic/internal.rs
@@ -337,7 +337,7 @@ fn add[](a: &AtomicRepr<Self>, v: Self::Delta) {
         /// Atomically updates `*a` to `(*a).wrapping_add(v)`, and returns the value of `*a`
         /// before the update.
         fn fetch_add[acquire, release, relaxed](a: &AtomicRepr<Self>, v: Self::Delta) -> Self {
-            // SAFETY: `a.as_ptr()` is valid and properly aligned.
+            // SAFETY: `a.as_ptr()` guarantees the returned pointer is valid and properly aligned.
             unsafe { bindings::#call(v, a.as_ptr().cast()) }
         }