Where can I find the APFS format documentation / volume specification?
I think Alsoft's use of the phrase APFS format documentation is just that - a phrase they've used rather than referencing an official title. 

The Apple File System Reference is almost certainly what they're referring to. This reference was publicly released on 17 September 2018, and Alsoft made that statement on 4 October 2018.

Prior to the Apple File System Reference, there was no consolidated documentation from Apple regarding APFS - at least not to this level of detail. In fact, the most comprehensive documentation until this was released was actually the result of private academic work in the computer forensics field.

As for Apple itself, we've had the following documentation:


Apple File System Guide - retired in June 2018
About Apple File System - first published in June 2018, but updated in October 2018 to replace references to HFS+ with HFS Plus instead
File System Programming Guide - last updated in April 2018


However, the Apple File System Reference was last updated in February 2019.