rust: num: bounded: add missing comment for always inlined function

rust: num: bounded: add missing comment for always inlined function

This code is always inlined to avoid a build error if the error path of
`build_assert` cannot be optimized out. Add a comment justifying the
`#[inline(always)]` property to avoid it being taken away by mistake.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20251208-io-build-assert-v3-7-98aded02c1ea@nvidia.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>