Future of typing_extensions.Doc

PEP-727 proposed a new Doc construct, but the PEP is unlikely to be accepted and will probably be withdrawn (https://discuss.python.org/t/pep-727-documentation-metadata-in-typing/32566/181).

That makes it so we have to figure out what to do with typing_extensions.Doc in the future. We don't have any previous cases where a feature was proposed in a PEP that didn't get accepted. We do have the precedent of typing_extensions.IntVar, which was added a long time ago without ever being proposed in a PEP. It is still in typing_extensions but undocumented.

I'm hesitant to remove importable objects from typing_extensions, because this library is so widely used and is a dependency of many other important libraries. If we remove something, we could end up breaking an important third-party library. Libraries could also end up pinning the version of typing-extensions they depend on, which is disruptive because it restricts users using that library from using new features of newer versions of typing-extensions.

Doc is also quite simple (a single class of a few dozen lines; no interaction with other features). Therefore, I don't see a big problem with keeping it around. However, we should mark it as deprecated in the documentation and probably move it into its own section.

首先查看 typing_extensions.Doc 的现有文档以及 issue 中链接的讨论，然后比较对 IntVar 的处理方式。当项目决定应如何处理 Doc，且文档反映了该决定（如果采用，还包括弃用和章节位置）时，该 issue 才算完成。