needless_doctest_main: false positive with proc-macro attribute

```console
$ cargo clippy -V
clippy 0.0.212 (ea85b4c 2020-02-01)
```

The `needless_doctest_main` lint triggers on this code:
```rust
/// ```
/// #[tokio::main]
/// async fn main() {
///     // ...
/// }
/// ```
```
Where the `fn main` is very much not needless :)