文档字符串。注意，是 __doc__ ， 前后各两个下划线。
一般而言，是对函数/方法/模块所实现功能的简单描述。但当指向具体对象时，会显示此对象从属的类型的构造函数的文档字符串。(示例见以下 a.__doc__)
>>> str.__doc__
"str(string[, encoding[, errors]]) -> str\n\nCreate a new string object from the given encoded string.\nencoding defaults to the current default string encoding.\nerrors can be 'strict', 'replace' or 'ignore' and defaults to 'strict'."
>>> import math
>>> math.__doc__
'This module is always available. It provides access to the\nmathematical functions defined by the C standard.'
>>> a = [1]
>>> a.count.__doc__
'L.count(value) -> integer -- return number of occurrences of value'
>>> a.__doc__
"list() -> new empty list\nlist(iterable) -> new list initialized from iterable's items"
为自定义的函数创建 __doc__ 的方法示例：
>>> def func():
"""Here's a doc string"""
pass
>>> func.__doc__
"Here's a doc string"
