Improve motivating example for Maybe

首先找到包含当前 Maybe 动机示例的文档部分；issue 中没有指定文件或测试。将该示例与所述的 Python 替代方案进行比较，并明确预期的使用场景。当文档能够呈现可信的动机，而不把含义不明确的 None 返回值当作常规错误处理时，即视为完成。

The current example motivating the use of Maybe is somewhat misleading because it solves a made-up problem:

Alleged original "python" code:

if user is not None:
     balance = user.get_balance()
     if balance is not None:
         credit = balance.credit_amount()
         if credit is not None and credit > 0:
             discount_program = choose_discount(credit)

Alleged "better" solution using Maybe:

discount_program: Maybe['DiscountProgram'] = Maybe.from_optional(
    user,
).bind_optional(  # This won't be called if `user is None`
    lambda real_user: real_user.get_balance(),
).bind_optional(  # This won't be called if `real_user.get_balance()` is None
    lambda balance: balance.credit_amount(),
).bind_optional(  # And so on!
    lambda credit: choose_discount(credit) if credit > 0 else None,
)

Usual python code solving this exact problem:

try:
    discount_program = choose_discount(user.get_balance().credit_amount())
except AttributeError:
    pass

The example is based on the very bad habit of signaling errors by return values, e.g. returning None.

No sane (python) developer would write a function that returns None in case of an error unless there is good reason for it, it is properly documented and returning None immediately and unambiguously tells the caller what went wrong. When exceptions occur, exceptions should be raised.

For example, credit_amount() returning None conveys no meaning at all. No credit? Credit amount == 0? Credit amount < 0? Raccoons taking over the world?

And even if one had to use flawed 3rd party code like this, there is a shorter and more concise version to handle this without Maybe.

I believe there is a legitimate use case for Maybe, but this is not it.