Rollup merge of #40783 - stepancheg:cursor-new-0, r=aturon

Document Cursor::new position is 0

... even if contained `Vec` is not empty. E. g. for

```
let v = vec![10u8, 20];
let mut c = io::Cursor::new(v);
c.write_all(b"aaaa").unwrap();
println!("{:?}", c.into_inner());
```

result is

```
[97, 97, 97, 97]
```

and not

```
[10, 20, 97, 97, 97, 97]
```