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 ?

從 Python Developer’s Guide 中連結的狀態鍵開始，並將其中的分支描述與所述的 docs-backport 問題進行比較。向相關 maintainer 確認 feature、prerelease、bugfix 和 security 分支上文件修正的核准規則；完成的標準是頁面明確說明文件變更可以 backport 到哪些分支，以及是否需要 RM 核准。