compiler/rustc_error_codes/src/error_codes/E0739.md

`#[track_caller]` must  be applied to a function

Erroneous code example:

```compile_fail,E0739
#[track_caller]
struct Bar {
    a: u8,
}
```

[RFC 2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md