Document type inference behavior in case type declaration has (not) initializer

In cases like this mypy doesn't narrow down the type in assignment:

x: Union[int, str] = 0
# type of x is Union[int, str] here!

Any assignment after the initial declaration will narrow the type down:

x: Union[int, str]
x = 0
# type of x is int here!

Document this behavior since it can be surprising. The motivation for this is that this behavior makes it easy to override the inferred type for a variable, which is sometimes useful.

使用 issue 中的两个示例作为文档来源：比较带有初始化器的声明与后续赋值，并解释不同的 narrowing 行为，以及为什么覆盖推断出的类型可能很有用。找到 mypy 文档中合适的部分，在其中添加说明，并验证两个示例及其推断出的类型都是准确的。