rust: num: bounded: clean __new documentation and comments

rust: num: bounded: clean __new documentation and comments

Following commit 3a1ec424dd9c ("rust: num: bounded: mark __new as
unsafe"), remove the redundant paragraph in the documentation of __new now
that the Safety section explicitly covers the requirement.

Additionally, add an INVARIANT comment inside the function body where
the Bounded instance is actually constructed to document that the type
invariant is upheld.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/CANiq72mUCUh72BWP4eD1PTDpwdb1ML+Xgfom-Ys6thJooqQPwQ@mail.gmail.com/
Signed-off-by: Shivam Kalra <shivamklr@cock.li>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260123132132.53854-1-shivamklr@cock.li
[ Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>