Update show-heartbeat documentation

[Osmose]

The show-heartbeat documentation is out-of-date with recent additions (such as the 24-hour delay between showing individual Heartbeat prompts. We should update it.

@andymikulski You're most familiar with the new options we've added to show-heartbeat, I imagine you're the best person for this issue, though it's not urgent or anything.

[shell1]

@shell1 good candidate to move this issue to bugzilla - firefox: normandy client and close out this issue.