--die-with-parent does not kill spawned processes unless --unshare-pid is set

Hello, I've been playing around with bubblewrap, and something that tripped me up is when I ran a program in `bwrap` with `--die-with-parent` that spawned one or more sub-processes, those sub-processes weren't getting killed. However when I added `--unshare-pid`, the child processes were properly killed. Note that **both** `--die-with-parent` and `--unshare-pid` had to be specified for bwrap to get the kernel to properly kill of spawned child processes.

You can see the tests I was running at https://git.kindrobot.ca/kindrobot/bwrap-dwp-test.

Is this the intended behaviour? If so, should something be added to the documentation under `--die-with-parent`?