Document default values in annotations

Hi,

I have found nothing in the documentation (and no test, but didn't search for too long).

Are there some docs about annotations that I can use without explicitly naming the parameters?
I guess it just injects an array with the key `"value"` but I am not to sure.

For reference, I mean code like this:

```php
/**
* @SomeAnnot("ohai")
*/
```

How is this unnamed parameter injected? It would be great if we could add a note about that on https://www.doctrine-project.org/projects/doctrine-annotations/en/latest/custom.html