with the recent launch of Wikifunctions, it might not be obvious for Wikipedia users they need not create a new login and could reuse the existing credentials instead.

so it'd be helpful to mention "You can use your global account" in the login page:
https://www.wikifunctions.org/w/index.php?title=Special:UserLogin

the situation is well documented elsewhere, it just could be made more obvious:
https://www.mediawiki.org/wiki/Help:Logging_in#Wiki_families_and_unified_login
https://meta.wikimedia.org/wiki/Help:Unified_login

please note this suggestion is not restricted to Wikifunctions,
it extends to all deployments of mediawiki in Wikimedia projects:
https://www.wikidata.org/w/index.php?title=Special:UserLogin
https://en.wiktionary.org/w/index.php?title=Special:UserLogin