Make RestartPreventExitStatus= documentation resemble SuccessExitStatus=

Make RestartPreventExitStatus= documentation resemble SuccessExitStatus=

The documentation for `RestartPreventExitStatus=` differs from that for `SuccessExitStatus=` in ways that are sometimes confusing (e.g. using `numeric exit codes` instead of `numeric termination statuses`), and other times plain incorrect (e.g. not mentioning `termination status names`, which I've just confirmed to work in systemd 255).

This patch modifies the documentation to be as similar as possible, so as to reduce the reader's cognitive load.