Schema description from docstring

Nobody has claimed this yet.

I wondered why the models in the overview at the end of the Swagger UI didn't contain a model description and went down the rabbit hole to find that it seems to be an [apispec issue](https://github.com/marshmallow-code/apispec/issues/208).

It seems the apispec project doesn't see a need to provide an easy way to add descriptions to a marshmallow schema class since it's already possible to do
```
class ASchema(Schema):
a = fields.Int(description="description for a field")

spec.definition('ASchema', schema=ASchema, description="description for a definition, or a schema")
```

What would be a clean way to apply this to flask-smorest? Or am I doing it all wrong and there is a better way to have a *components.schemas[].description* added to in the resulting openapi.json?

I'd love if the class docstring could be used for this.

No file or test is named. Start by tracing how marshmallow schemas are passed to apispec and how the OpenAPI components.schemas entries are generated; the work is done when a schema class docstring produces a components.schemas[].description in the resulting openapi.json, with coverage for that behavior.

Written by the indexing model from the issue text.