Hey Stefan,
we're in the middle* of replacing the existing API with an altogether rewritten RESTful API. The new API is designed to be unified in contrast to the current, organically-grown API parts scattered between Telnet (FTL) and PHP (doing some proxying, some post-processing).
The current development version of FTL for the v6.0-era lives in branch new/http. It is currently still possible to you check out this branch and go back without any consequences (still backwards-compatible). Once you have checked out FTL, you will get access to the fully embedded documentation of the version you're currently running. Furthermore, the documentation is automatically updated after each commit on our development server:
https://ftl.pi-hole.net/new/http/docs
However, mind that the embedded documentation allows you true interaction with the API, e.g. you can add/modify/remove domains from your Pi-hole while learning how to use it (using the "Try" feature), like querying the current statistics, etc. (live data, not some demo data):
*) About 50% of the work is done.