The CreateSession description states that it should never be called from the clients, but instead be handled through PAM, so assume this is why they don't describe the arguments for this one call, while all the other method arguments and return values are described under 'Methods'.
What I mean specifically is that they document the client interface only, not the full interface needed for a complete replacement.
If you want to re-implement logind you will invariably read the logind source code anyway, where the call arguments are described.
You said there been a complaint about this, was there a discussion you can point me to ?
1) Does this mean that you consider software's source code to be adequate documentation of an API and its behavioral contracts?
2) There's this saying that goes something like "You never know that you have a good API until you've created three independent implementations of it.". It's very difficult to create an independent implementation of an API if the primary documentation of important parts of that API's behavior is the source code that implements that API.
If this fails the test, I suggest that it is a marvelous failure which every project should strive to emulate. To suggest that systemd is poorly documented indicates an insufficient exposure to the sad reality of most Open Source projects where "our unmaintained, outdated, bug riddled code base is the documentation" is the best case answer to the question of documentation.
There are a lot of praiseworthy things about systemd and its approach to documentation. The blog posts are definitely one of them. That doesn't mean everything is praiseworthy, that there's no room for improvement, or that complaints about things not being documented are illegitimate or inappropriate.