Add lint for `bare_must_use` - `#[must_use]` without a note

What it does

Stylistic lint that warns on bare #[must_use] attributes without a #[must_use = "note of some sort"]. Was actually surprised it didn't exist already.

Advantage

• For the types of projects where maximum documentation is desired (and clippy is already turned up to 11), this adds another means by which code intent can be captured.
• Adds a note: clause in the compiler error that explains this intent at the callsite.

Drawbacks

None that I can think of. Probably shouldn't be enabled by default though.

Example

#[must_use]                                 // <- would warn if `clippy::bare_must_use = "warn"`
fn get_foo() -> usize {
    // ... some expensive operation ...
    42
}

Could be written as:

#[must_use = "getting foo is very expensive"]
fn get_fo() -> usize {
    // ... some expensive operation ...
    42
}

Comparison with existing lints

There exist already a bunch of #[must_use]-related lints, but seemingly none of them check adding the notes.

Additional Context

This is a safety-critical project whereby as much documentation, verbosity in linting and testing, etc. is preferred and desired. I didn't even know #[must_use] took a note string until I saw it in the standard library sources a few minutes ago, and discovered clippy didn't have a lint to enforce that as I think it's better (and more enforceable) than asking people to add a // reason comment next to the #[must_use] in all cases.