rust: num: bounded: clean __new documentation and comments

Following commit 3a1ec42 ("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>

Author: Shivam Kalra

rust/kernel/num/bounded.rs

@@ -282,9 +282,6 @@ where
     /// All instances of [`Bounded`] must be created through this method as it enforces most of the
     /// type invariants.
     ///
-    /// The caller remains responsible for checking, either statically or dynamically, that `value`
-    /// can be represented as a `T` using at most `N` bits.
-    ///
     /// # Safety
     ///
     /// The caller must ensure that `value` can be represented within `N` bits.
@@ -297,6 +294,7 @@ where
             assert!(N <= T::BITS);
         }
 
+        // INVARIANT: The caller ensures `value` fits within `N` bits.
         Self(value)
     }