…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

…, r=Nemo157,jyn514

Fix item name display on mobile

Fixes rust-lang/docs.rs#1200

![Screenshot_20201211-200931](https://user-images.githubusercontent.com/3050060/101944457-0c06eb00-3bed-11eb-8f63-a4d4fd3cbb56.jpg)
![Screenshot_20201211-195846](https://user-images.githubusercontent.com/3050060/101944459-0d381800-3bed-11eb-91ff-815a2af7ca72.jpg)

cc `@jyn514`

r? `@Nemo157`

… r=jyn514

Refactor test_lang_string_parse to make it clearer

Follows rust-lang#79454 (comment)

A small PR made to refactor a test in rustdoc that was becoming unwieldy.

``@rustbot`` label T-rustdoc
r? ``@jyn514``