Document how end_time and max_end_time behave

[PeterJCLaw]

These are very non-obvious to choose.

The alternative would be to make end_time do the more obvious thing, however that would be breaking.

Briefly, within a period schedule:

• end_time is the latest that a match slot can begin, before any delays are accounted for (and thus determines which matches will attempt to be scheduled as part of the period)
• max_end_time is the latest a match slow can begin, once delays are accounted for (and thus determines which matches are actually fit into the period)

[PeterJCLaw]

This was somewhat achieved by PeterJCLaw/dummy-comp@1c264fe