`graphql_schema` introspection doesn't output docstrings descriptions

**Note: for support questions, please use stackoverflow**. This repository's issues are reserved for feature requests and bug reports.

* **What is the current behavior?**

Running `manage graphql_schema` to export the `schema.graphql` doesn't output the description for types and fields. I've only tested it when the description is described via docstrings, e.g.:

```
class SomeType(graphene.ObjectType):
"""
Some description
"""

field = graphene.String()
```

but I suspect it also doesn't work when using `description=` or the `Meta` class as described here:

* **If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem** via
a github repo, https://repl.it or similar.

- Create a schema with at least one type with a description (and/or a field description)
- Run the `graphql_schema` introspection command to export a `schema.graphql` file

* **What is the expected behavior?**

The schema is outputted including field and type descriptions.

* **What is the motivation / use case for changing the behavior?**

No type or field descriptions are exported.

* **Please tell us about your environment:**

- Version: ^2.13.0
- Platform:

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow)

Reproduisez le problème en définissant un type et un champ Graphene avec des descriptions dans la docstring, puis exécutez la commande `manage graphql_schema` pour exporter `schema.graphql`. Comparez le schéma exporté avec les descriptions attendues du type et du champ ; c’est terminé lorsque les deux descriptions sont incluses dans la sortie et que le comportement de `description=` et de `Meta` a également été vérifié, le cas échéant.

Rédigé par le modèle d'indexation à partir du texte de l'issue.