Compare PageImage's API help with, e.g. linkshere API help. PageImages doesn't have a link to docs, similarly in ApiSandbox its Documentation doesn't have a (read more) link.
The fix is for ApiQueryPageImages.php to implement getHelpUrls(). It should probably link to https://www.mediawiki.org/wiki/Special:MyLanguage/Extension:PageImages#APIs ... though no other extension prepends Special:MyLanguage to its help URL path.