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PEP-727 proposed a new Doc construct, but the PEP is unlikely to be accepted and will probably be withdrawn (https://discuss.python.org/t/pep-727-documentation-metadata-in-typing/32566/181).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

Start by reviewing the current documentation for typing_extensions.Doc and the discussion linked in the issue, then compare the treatment of IntVar. The issue is complete when the project has decided how Doc should be handled and the documentation reflects that decision, including deprecation and section placement if adopted.