Unneeded `rust` as language in `rustdoc`'s documentation examples

What it does

In a documentation example, when processed by rustdoc, it is strictly equivalent to write rust or not (https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html).

Thus it would be nice to have a lint that suggest the removal of the unneeded rust as language for the example.

Some projects may prefer the opposite, i.e. always writing rust, thus the equivalent lint for that may be a good idea as well. Either way, consistency is typically an improvement.

Advantage

• Less busy, and more consistent, source code.
• When contributing to a project, there is no need to remember if it is supposed to be written or not.

Drawbacks

• It may be slightly harder to copy the code into a non-rustdoc system.

Example

/// ```rust
/// let x = 5;
/// ```

Could be written as:

/// ```
/// let x = 5;
/// ```

Start with rustdoc's documentation-tests guidance linked in the issue and compare how language tags are processed. Decide which consistency policy the lint should enforce, then verify that the lint identifies redundant rust tags without affecting other documentation examples; the issue does not name implementation files or tests.