[spec] Overload processing order

It's my understanding that overloads are processed in order. I.e. given the following overloads:

@overload
def foo(x: int) -> None: ...
@overload
def foo(x: int | str) -> None: ...
@overload
def foo(x: str) -> None: ...

When called with an int, the first overload is matched, when called with a str, the second is matched. The third is never matched. (Not going into what happens when called with int | str.) Currently, this behavior is not mentioned in the spec.

首先定位描述重载解析的 typing specification 部分，并将其当前措辞与 issue 中依赖顺序的示例进行比较。只有当 specification 清楚说明重载如何按顺序处理，并解释所示的 int 和 str 情况时，这项工作才算完成。