Document nested doc comments in grammar

Currently, the grammar doesn't really specify that ron supports nested block comments:
https://github.com/ron-rs/ron/blob/master/docs/grammar.md?plain=1#L21

But it does:
```rs
    let _: ron::Value = ron::from_str(r#"
        Something(
            /* /* test */ */
            a: b
        )
        "#).unwrap();
```