GraphQLUnionType documentation is incorrect

GraphQLUnionType currently gives the following example of its usage:

```python
class PetType(GraphQLUnionType):
    name = 'Pet'
    types = [DogType, CatType]

    def resolve_type(self, value, _type):
        if isinstance(value, Dog):
            return DogType()
        if isinstance(value, Cat):
            return CatType()
```

I believe this documentation is incorrect; `PetType()` can't be instantiated since GraphQLUnionType's `__init__` takes name and types as required parameters.

Calling `PetType(name=PetType.name, ...)` is obviously terrible, and implementing an empty `__init__` which doesn't call `super()` feels unsafe.

Also, even if there's a way to define a UnionType via inheritance which I'm missing, I don't see why GraphQLUnionType example should be different from GraphQLInterfaceType example, which uses the normal function call API.

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