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

[woodruffw]

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:

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

[rust-highfive]

r? @dtolnay

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[dtolnay]

Thanks!

[dtolnay]

@bors r+ rollup

[bors]

📌 Commit d986924 has been approved by dtolnay