fix description for parameters in OpenAPI 3

[fMeow]

In OpenAPI3, desired output for query parameters is like this:

parameters:
- description: Password
  in: query
  name: password
  required: true
  schema: 
    type: string

However, only properties in, name and required are defined in the top level, while all other properties is moved under property schema, which conflicts with OpenAPI 3 specification.

Current output:

parameters:
- in: query
  name: password
  required: true
  schema: 
    type: string
    description: Password

So before moving all properties to schema, it's reasonable to check for description property and define description in top level of parameter object.

[sloria]

Thanks again @Guoli-Lyu . Can you add a test for this, too? Once that's added, this can be merged.

[fMeow]

I add test for descriptions in parameters and also requestBody, which in OpenAPI 2 is like this:

parameters:
- in: path
  name: name

@sloria

[sloria]

Looks good. Thanks for adding the test. Just some minor revisions to the docs needed. I can make those myself before I merge.

[sloria]

Minor revisions:

For OpenAPI 3, only "query", "header", "path" or "cookie" are allowed for the location of parameters. "requestBody" is used when fields are in the body.

This function always returns a list, with a parameter for each included field in the :class:`Schema <marshmallow.Schema>`.