Add docs for `position!() (@) position!()` ?

```rust
rule x() -> () = precedence! {
    a:position!() x:(@) b:position!() { dbg!(a, b); }
    --
    a:(@) "+" b:@ {}
    --
    "x" {}
}
```

It seems that `a:position!() x:(@) b:position!()` is not mentioned in the document, but it is very useful.
Can it be explained in the document?
