flask-apispec==0.8.7 : how to use apispec doctrings to populate the swagger documentation info?

Hello,
i have a very simple endpoint like this:

```
app = Flask(__name__)
docs = flask_apispec.FlaskApiSpec(app)

@app.route('/')
def main():
"""top-level stuff
---
get:
summary: whatever
description: double whatever
parameters: None
responses:
200:
description: all good here
"""
return "all good here\n"

docs.register(howdy)
```

but the swagger is never generated from the docstring. i can _at least_ produce the description via `@flask_apispec.annotations.doc(description='top-level stuff')` but i couldnt find a way to populate the other information in the swagger interface with this decorator.

how do i use the apispec docstring with flask_apispec?

i have `apispec==3.3.0` installed in this venv, in case it matters

thanks!

Reproduce the example using FlaskApiSpec(app), the endpoint docstring, docs.register, and apispec==3.3.0; begin by checking how FlaskApiSpec processes docstrings versus annotations. Confirm whether the requested Swagger fields are supported, and document the working usage or the limitation.

Written by the indexing model from the issue text.