Missing examples / tests for --userns2 option

Trying to get a grasp on the nested namespaces and how to enter those with --userns2
There are no examples or tests I can find

For example when I run
```sh
bwrap --unshare-user --dev-bind / / --tmpfs /tmp /bin/bash
```
I believe this creates a nested user namespace, I can see a complete namespace set (time,cgroup,uts,ipc,net,user,mnt,pid) using lsns
while if I do lsns on the host there are only 2 namespaces for the process (user,mnt)

How can I make this work?
```sh
bwrap --userns 11 --userns2 12 --dev-bind / / /bin/bash 11