Unlike eg. ATA and SCSI, the NVMe specs are freely available to the public. They're a little more complicated to read now that the spec has been split into a few modules, but finding the descriptions of all the optional features isn't too hard.
The nvme-cli tool and its documentation is written with the assumption that the user is at least somewhat familiar with the NVMe spec or protocol itself, because a large part of the purpose of that tool is to expose NVMe functionality that the OS does not currently understand or make use of. It's meant to be a pretty raw interface.
