shared: document allocateUnit limitation on D-Bus fallback path

shared: document allocateUnit limitation on D-Bus fallback path

The D-Bus registration methods (RegisterMachineEx, RegisterMachineWithNetwork)
do not support the allocateUnit feature that the varlink path provides.
When varlink is unavailable and registration falls back to D-Bus, machined
discovers the caller's existing cgroup unit instead of creating a dedicated
scope. Callers that skip client-side scope allocation (relying on the
server to do it via allocateUnit) will end up without a dedicated scope
on the D-Bus fallback path.

Document this limitation at the fallback site so callers are aware.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>