Improve documentation of MapReducer's timestamp setter

[SlowMo24]

We implement our own ISO 8601 String parser (is there really no default parser out there?). When supplying a Date this is transformed to a Timestamp https://github.com/GIScience/oshdb/blob/eac6d34ac893615c65fe3f30a389f70681c74e33/oshdb-util/src/main/java/org/heigit/bigspatialdata/oshdb/util/time/ISODateTimeParser.java#L59 using the 'start of the day' i.e. ...T00:00:00Z

This should be documented in https://github.com/GIScience/oshdb/blob/3b46a1390e33d920d9ed9eddabed56a661997d9e/oshdb-api/src/main/java/org/heigit/bigspatialdata/oshdb/api/mapreducer/MapReducer.java#L303-L316

[SlowMo24]

fix when addressing #169 , also related: #259 et al

[tyrasd]

How else did you expect the transformation to be done if the time part of a date-time string was not provided? Noon (12:00:00) or end of day (23:59:59)?

[SlowMo24]

I had no expectation to the implementation (although date] and date[ seem equal opponents and I prefer the current implementation) but rather to the documentation ;-)

[tyrasd]

ok. It's about improving the documentation… I changed the title of the ticket accordingly.

[tyrasd]

sorry, misclick

[Hagellach37]

@tyrasd will work on this.