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Compare PageImages' 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.