The diff explains the problem. Another page in the docs says "ask the wiki owner": seriously?
The endpoint URL must be added in some place where you'll be able to find it even if you don't know the script path or the index.php URL. It should be very easy to put it in a comment somewhere in [[Special:Version]], for instance.
Note that of course if you know the API endpoint you can get the index.php URL via the API itself.