docs(calendar): clarify pastDaysCount and broadcastPastEvents (#357)

Resolves #290

- Clarify that pastDaysCount only affects broadcasts, not display
- Explain that broadcastPastEvents requires pastDaysCount to be set
- Fix incorrect reference to maximumNumberOfDays (should be pastDaysCount)
- Emphasize that calendar module never displays past events

The previous documentation caused confusion as users expected
pastDaysCount to show past events in the calendar display, when
it only affects the CALENDAR_EVENTS broadcast to other modules.

Author: KristjanESPERANTO