The documentation does still need a lot of work, and there could be a lot more info on module. The aim is for all of that to be a lot more complete in the future, but there are very few contributors and it all takes time.
Comparing it to WHMCS isn't really fair for the reason given in the FAQ - Here
With that said there is a basic guide on module creation - Here
There is also a guide to the API that may help - Here
The documentation, like FOSSBilling itself is open source and anyone can contribute to it. So, if you do decide to work on a module and think of anything that should be added to the docs, please do feel free to hit 'edit' and submit a PR to update the docs as you go.