Rollup merge of rust-lang#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]
```

src/libstd/io/cursor.rs

@@ -89,6 +89,10 @@ pub struct Cursor<T> {
 impl<T> Cursor<T> {
     /// Creates a new cursor wrapping the provided underlying I/O object.
     ///
+    /// Cursor initial position is `0` even if underlying object (e.
+    /// g. `Vec`) is not empty. So writing to cursor starts with
+    /// overwriting `Vec` content, not with appending to it.
+    ///
     /// # Examples
     ///
     /// ```