Support for dataclass docstring

[diogo-rossi]

This ideia is similar to #234, but in more specific way that could seize existent code used for def.

Example:

@dataclass
class Person:
    """Description of the Person dataclass

    Attrs:
        name (str): The name of the person
        age (int): The age of the person
        temp (float, optional): The body temperature. Defaults to 36.
    """
    name: str
    age: int
    temp: float = 36.0