Subcommands missing documentation

[Bluenix2]

Summary

While trying to help people in the help channels I realised that there is no documentation on subcommands. I found some in the FAQ about how to do subcommands but no full documentation like checks and commands have here. This is probably needed as all other aspects of the commands extension have great and well-done documentation. Of course you have the API Reference but I still think that this is needed.