Document better what "internal types" are

There is this notion of "internal types" in the graphql spec: They are mentioned the first time in "Result Coercion" section of [Scalars](https://graphql.github.io/graphql-spec/draft/#sec-Scalars).

It is never explicitly stated how these internal types relate to the "external/visible" types of GraphQL and how they relate to the serialization.

For example:
We have the Float scalar type of GraphQL and also an internal type of Float which is different, which is then maybe serialized to String.