Document that we can't use new stdlib symbols until added to type-checkers

https://github.com/python/typeshed/pull/11890#discussion_r1597530831
An unwritten convention (because we've been bitten by it a few times in the past), is to not use newly added symbols from the stdlib stubs until all supported type-checkers (pyright, mypy, pytype??, realistically we're always waiting on mypy release because pyright releases so often) updated their vendored version of typeshed.

This is a suggestion to add this convention to the contributing guideline in a more official way, and to teach contributors about this unobvious gotcha.

Relates to #5835

先阅读贡献指南和 typeshed PR 11890 中的讨论，尤其是其中链接的评论，以了解现有约定。记录以下内容：新添加的 stdlib stub 符号应等待受支持的 type-checker 更新其 vendored typeshed 版本；当该指导对 contributors 清晰明确且相关指南已更新时，即视为完成。