popen: clarify group signaling details

Even when ..._SETSID and ..._GROUP_SIGNAL are set, we unable to safely
kill a process group after the child process we spawned becomes died. So
we don't do that.

The behaviour seems to be indefeasible part of Unix process group
design. The best that we can do here is describe those details in the
documentation comment.

NB: It seems that pid namespaces allow to overcome this problem, however
it is the Linux specific feature, so we unlikely will use them.

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>