Improve #[from] documentation example.

Hey folks,

There is an issue that took me a while to understand, and it's that in the lib doc is not very clear how to display the error that is being derived with `#[from]` macro.

The actual example is e.g.

```Rust
#[error("data store disconnected")]
Disconnect(#[from] io::Error)
```

Despite there is also an example of how to use from attribute over a struct field e.g.

```Rust
#[derive(Error, Debug)]
pub enum MyError {
Io {
#[from]
source: io::Error,
backtrace: Backtrace,
},
}
```

None of these examples tells you can do it e.g.

```
#[error("data store disconnected: {0}")]
Disconnect(#[from] io::Error)
```

Would be great to have it in the doc.

Thanks!