[FLINK-4121] Add timeunit (ms) to docs for timestamps and watermarks

[powibol]

This PR inserts an explicit specification of the timeunit used in watermarks and timestamps in the documentation. (According to FLINK-4121).

I'm teaching a class where students are supposed to implement Flink streaming programs. It seems to be unclear for them, that the time unit of timestamps and watermarks is milliseconds. They just see TimeWindow.seconds(...) or something similar and assume the timestamps to be of the same unit (seconds) which is obviously not the case. I just checked the documentation and there is no explicit specification of the time unit used for timestamps. After short talk with Aljoscha Krettek, I'll open a mini-PR to address this issue.

@aljoscha Please let me know in case you need anything else to merge this.

[aljoscha]

I think it would be better to only have one place in the doc where we say that timestamps/watermarks are in milliseconds (with a red warning box, so it's visible). I can change it while merging