Update marshmallow plugin documentation for custom types
Update marshmallow plugin documentation for custom types

Now that the OpenAPI type for a custom field uses the class hierarchy to work out
what OpenAPI type to use, the documentation needs updating. This change expands
on how to use the map_to_openapi_type decorator and explains that cases where it
is no longer necessary.

marshmallow-code/smore,marshmallow-code/apispec