…side-effects, r=dtolnay

doc(array,vec): add notes about side effects when empty-initializing

Copying some context from a conversation in the Rust discord:

* Both `vec![T; 0]` and `[T; 0]` are syntactically valid, and produce empty containers of their respective types

* Both *also* have side effects:

```rust
fn side_effect() -> String {
    println!("side effect!");

    "foo".into()
}

fn main() {
    println!("before!");

    let x = vec![side_effect(); 0];

    let y = [side_effect(); 0];

    println!("{:?}, {:?}", x, y);
}
```

produces:

```
before!
side effect!
side effect!
[], []
```

This PR just adds two small notes to each's documentation, warning users that side effects can occur.

I've also submitted a clippy proposal: rust-lang/rust-clippy#6439

…laumeGomez

Rollup of 3 pull requests

Successful merges:

 - rust-lang#79918 (doc(array,vec): add notes about side effects when empty-initializing)
 - rust-lang#79936 (Fix item name display on mobile)
 - rust-lang#80013 (Refactor test_lang_string_parse to make it clearer)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup