Documentation mistakenly using `///` instead of `//!`

What it does

In the Linux kernel we had some documentation added with /// instead of //!, which meant the docs applied to a use line and thus silently discarded.

Advantage

Documentation is not discarded silently.

Drawbacks

False positives when someone does want to document an item that follows. However, if it is something like a private use, it is likely it was a mistake. Thus the lint should limit itself to such cases.

Example

Both

//! Title.
/// Long...
///
/// ...description.
use ...;

and

//! Title.

/// Long...
///
/// ...description.
use ...;

should probably have been written as

//! Title.
//!
//! Long...
//!
//! ...description.

use ...;

Start by comparing the two Rust examples in the issue, focusing on outer documentation attached to a use line and the intended inner-documentation form. Define how the lint distinguishes likely mistakes from intentional item documentation, including the private-use limitation. Done means the discarded-documentation case is detected without the described false positives.