@ that makes implicit arguments explicit

**What question should the reference manual answer?**

What does `@` mean in `@List.nil Nat`?

**Additional context**

It might be there, but I couldn't find it. If it is there, the request is to make it more findable:

* When searching for `@`, it currently doesn't show this usage of `@`
* I also couldn't find it in sections 10.1-10.4, where I would have expected to find it.
