man: improve documentation for RestartSteps (#40879)

man: improve documentation for RestartSteps (#40879)

I found the existing explanation of RestartSteps to be simply
impenetrable. Even providing the full docs context to several of our new
AI overlords resulted in wildly different (and completely incorrect)
explanations of the final restart intervals.

Digging through the code, I found the restart delay computation in
`service_restart_usec_next` in `src/core/service.c`.

I've updated the documentation for RestartSteps with an example,
suggested value range and a detailed enough explanation that accurately
describes the current behavior.

(cherry picked from commit 4724cf5526b5c6a08267d1b1bb23e962ebe60521)
(cherry picked from commit 7b1ea2374aa9834e41d4e444f54e25cc5bd7f7b9)
(cherry picked from commit 4479c8a0fa937f5e7dc0f519f5b184e3ad86f11a)