When you need to accept some unstable packages, there's fair amount on masking, and various features, but the simplest solution to avoid ~arch packages from being downgraded, is to emerge -U. There's no mention in manual. A short one or two line note after explanation of ACCEPT_KEYWORDS example would suffice and save many users time and trouble.
Created attachment 20777 [details, diff] Patch to portage manual This patch adds a paragraph right after "emerge --update world" to inform the users that, if they don't want packages to be downgraded, to use "emerge --upgradeonly world". If a docdev can review?
patch looks good
Committed.