rust: sync: atomic: Update documentation for `fetch_add()`

The documentation for `fetch_add()` does not indicate that the original
value is returned by `fetch_add()`. Update the documentation so this is
clear.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Link: https://patch.msgid.link/20260220-atomic-sub-v3-2-e63cbed1d2aa@kernel.org