I have to disagree, k8s is extensively documented and the reference docs and APIs are easily accessible. K8s at its core is a customizable, extensible, dynamic API server, with a focus on containerization. It's built with scale and customization in mind, you're not supposed to use it only for running a few containers. I've worked for people that use it to manage VMs with custom controllers. You can change pretty much anything and fit it to your needs. All this with defined, somewhat opinionated sane defaults and conventions

Poorly documented? Surely you jest, the K8s documentation is _excellent_.

For basically every core resource type, there’s a user-guide, examples, a tour of it, in-depth docs and then the api docs themselves.