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