[FLINK-9107][docs] document timer coalescing for ProcessFunction

[NicoK]

What is the purpose of the change

In a ProcessFunction, registering timers for each event via ctx.timerService().registerEventTimeTimer() using timestamps like ctx.timestamp() + timeout will get a millisecond accuracy and may thus create one timer per millisecond which may lead to some overhead in the TimerService.

This problem can be mitigated by using timer coalescing if the desired accuracy of the timer can be larger than 1ms. A timer firing at full seconds only, for example, can be realised like this:

long coalescedTime = ((ctx.timestamp() + timeout) / 1000) * 1000;
ctx.timerService().registerEventTimeTimer(coalescedTime);

As a result, only a single timer may exist for every second since we do not add timers for timestamps that are already there.

Please note that this PR includes #5788 and should also be merged into 1.3 and 1.4 docs to which it applies as well.

Brief change log

• document timer coalescing for ProcessFunction

[NicoK]

Actually, more advanced schemes using current watermark + 1 (which fires with the next watermark) for the event time timer should also go into the documentation. I'll extend the PR ...

[aljoscha]

typo: coalesc -> coalesce

[NicoK]

thanks - fixed and extended with an example for "schedule with the next watermark" for event-time timers

[twalthr]

Thank you @NicoK. I will merge this...