Improve ContextVar in closures documentation

Documentation

In the [documentation for contextvars.ContextVar], there is the following warning:

Important: Context Variables should be created at the top module level and never in closures. Context objects hold strong references to context variables which prevents context variables from being properly garbage collected.

@kumaraditya303 and I talked about it and he claimed that this is not the case any more (that was at least my understanding) and ContextVars can be defined as class/instance attributes for example. If that's the case, we should probably update the docs. Otherwise, it might make sense to emphasize on it even more, now that ContextVars will be even more widely used due to free-threading.

從 contextvars.ContextVar 的文件開始，確認關於閉包和垃圾回收的警告是否仍然準確，包括類別屬性和執行個體屬性。文件清楚反映已確認的行為和指引時即可完成。