There is no comprehensive and up-to-date HTTP API documentation. We do have:
but it's outdated.
You need to look at the source
https://github.com/pi-hole/AdminLTE/blob/6c320a42574f9f0944ded71d84b7331557dbc6ea/api.php
https://github.com/pi-hole/AdminLTE/blob/6c320a42574f9f0944ded71d84b7331557dbc6ea/api_FTL.php
P.S. We are working on Pi-hole v6, which should have a new, RESTful API. To have a look at the current state: https://ftl.pi-hole.net/new/http/docs/#overview