[Rustdoc] Show #[must_use] on functions

[Havvy]

As it's important information about the function, Rustdoc should show that the result of the function must be used.

[GuillaumeGomez]

Adding it.

[GuillaumeGomez]

Hum actually, how would the function show that the returned type should be used?

cc @rust-lang/docs

[frewsxcv]

Some history: #37012

[QuietMisdreavus]

I imagine, for bare functions, the #[must_use] attribute would be shown the same way as attributes are shown on structs. The problem comes in on associated methods on structs and traits. They don't have the same visual language as bare functions do, but they can get #[must_use] attributes the same way.