Here
In section
6. Entry points to kernel-md
6.4 Document API V2 (Java)
it says
The very latest version of this guide (as a raw .adoc file) can be found [in the kernel repository]
but that “kernel-repository” link is unfortunately a) broken and b) points to internal bitbucket
I managed to find the docs (I think) in mgm wiki at /confluence/display/A12/Document+API+V2 but of course that also is only visibly internally.
Also, it is unfortunate that the previous section 6.2 about document deserialization still uses the deprecated V1 api.
I think that should be clearly marked as DEPRECATED and moved to after the V2 API section.