Improve documentation of external API endpoints

[ahl]

With #640 we now have both summary and description fields populated in the OpenAPI output. This has made more apparent that our documentation could be more expansive. Perhaps this issue is too broad, but it might make sense for someone at some point to take a big swing through all the endpoints to properly document them. If there is some automation we can put in place (beyond the extant generation of OpenAPI spec and hope that code reviewers maintain a more loquacious standard of documentation) we should do that as well.