Native documentation should describe allowed types

[osa1]

Native documentation currently has no mentions of the types allowed in the function type.

Currently the only way for a user to find out the types is by looking at the example in the documentation:

@Native<Int64 Function(Int64, Int64)>()
external int sum(int a, int b);

And try to figure out what about Int64 makes it allowed in a @Native<...> (probably implementing NativeType), and find the other types with the same property (probably check out NativeType documentation to see implementers).

[dcharkes]

Correct, anything subtyping NativeType.

https://dart-review.googlesource.com/c/sdk/+/328260