Describing requestBody parameters

OpenAPI 3.0 supports the description field in the requestBody itself, the requestBody schema, and individual schema fields.

openapi: 3.0.2
...
paths:
  /login:
    post:
      requestBody:
        required: true
        description: A JSON object containing the username and password  # <----
        content:
          application/json:
            schema:
              type: object
              description: An object containing the username and password  # <----
              properties:
                username:
                  type: string
                  description: Your username  # <----
                password:
                  type: string
                  description: Your password  # <----
              required:
                - username
                - password
      responses:
        200:
          description: Successful login

Please see Describing Request Body for more examples.