README.Debian: document KillUserProcesses behavior in Debian

Upstream doesn't want to adjust documentation according to build configuration, but only document upstream defaults (see https://github.com/systemd/systemd/issues/4705).

Though patching man/systemd-run.xml downstream within Debian means that we'd have to carry and update it for possibly eternity.

So instead document the Debian default in README.Debian.

Closes: #904918, #827159