docs: add comment about requiring the mount hierarchy to be mounted MS_SHARED

docs: add comment about requiring the mount hierarchy to be mounted MS_SHARED

This has been tripping up container manager people. let's document this
explicitly.

(Note that the container interface could really use some updates, i.e.
it was written before a time where cgroup namespacing was a thing. But I
am too lazy to fix that now, so let's just add this once facet.)