Clarify where docs fixes can be backported

The status key for branches has lists of changes that can go in, and the lists look exclusive:

feature:

• new features, bugfixes, and security fixes are accepted.

prerelease:

• feature fixes, bugfixes, and security fixes are accepted for the upcoming feature release.

bugfix:

• bugfixes and security fixes are accepted, new binaries are still released. (Also called maintenance mode or stable release)

security:

• only security fixes are accepted and no more binaries are released, but new source-only versions can be released

end-of-life:

• release cycle is frozen; no further changes can be pushed to it.

This makes it a bit awkward to backport docs changes, especially now that the lists are included in the tag descriptions.

AFAIK, docs fixes don't need RM approval in feature, prerelease and bugfix branches. (Security branches are locked so any change needs the RM in the loop.)
Is that right, @pablogsal @Yhg1s ?

从 Python Developer’s Guide 中链接的状态键开始，并将其中的分支描述与所述的 docs-backport 问题进行比较。向相关 maintainer 确认 feature、prerelease、bugfix 和 security 分支上文档修复的批准规则；完成的标准是页面明确说明文档更改可以 backport 到哪些分支，以及是否需要 RM 批准。