Last modified: 2014-08-10 11:38:30 UTC
In Special:ApiSandbox and in http://en.wikipedia.org/w/api.php, the help for query meta=notifications and for action=echomarkread is a link to https://www.mediawiki.org/wiki/Echo_(notifications)/API , but the page doesn't exist.
I couldn't find a page with more information about the Echo API, and getHelpUrls() in api/ApiEchoNotifications.php which returns this URL dates from 2012, so this page was probably never written.
Should this actually be "Echo API lacks documentation"
https://www.mediawiki.org/wiki/Echo_%28Notifications%29 looks more like a braindump than structured documentation. The api (https://www.mediawiki.org/w/api.php) links to https://www.mediawiki.org/wiki/Echo_%28notifications%29/API for documentation, but that page doesn't exist.
This documentation needs a bit of wikilove.