man: document that "systemd-run -M" propagates exit codes, and "machinectl shell" does not

This adds a brief explanation, suggesting the use of "systemd-run -M" to
acquire exit status/code information for the invoked process.

My original plan was to propagate the exit code/status in "machinectl
shell" too, but this would mean we'd have to actively watch the shell's
runtime status, and thus would need full, highly privileged and
continious access to the container's system manager, the way
"systemd-run" does it. This would be quite a departure from the
simplistic, low-priviliged OpenShell() bus call implementation of the
current code, that really just acquires a PTY device with a shell
connected.

Moreover it would blur the lines between the two commands even further,
which I think is not desirable. Hence, from now on:

    "machinectl shell" is the full-session, interactive shell for human
    users

    "systemd-run -M …" is the low-level tool, that supports
    on-interactive mode, and is more configurable and suitable for
    streaming.

Fixes: #4215

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