Add explicit documentation regarding the ordering of Union types for serialization/deserialization

[john-bodley]

I was wondering whether there was merit in adding more documentation regarding the Union type, specifically related to the order of types is relevant—this is somewhat counter intuitive as the term union applies to set theory where sets are unordered.

For example say you have a field which could be a bool or int, then the following snippet,

from dataclasses import dataclass
from typing import Union

import marshmallow_dataclass


@dataclass
class Foo:
    bar: Union[bool, int]


foo_schema = marshmallow_dataclass.class_schema(Foo)()
foo = foo_schema.load({"bar": 1}) 

returns Foo(bar=True) rather than Foo(bar=1). Note for this specific example Union types aren't desirable given that if you switched to Union[int, bool] True would be deserialized to 1. Simply using Any seems preferred.

[lovasoa]

Yes, you are right, the documentation is not very clear about how Unions work. PR welcome !