`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)

docstring による説明を持つ Graphene の型とフィールドを定義して問題を再現し、`manage graphql_schema` コマンドを実行して `schema.graphql` をエクスポートします。エクスポートされたスキーマを、型とフィールドに期待される説明と比較します。出力に両方の説明が含まれ、該当する場合は `description=` と `Meta` の動作も確認できれば完了です。

索引モデルが issue の本文から書いたものです。