Add clarification about use of Time_format

Specifying `isoUtcDatetime` or `UTC:` prefix in here causes display issues. Some (but not all!?) datetimes are adjusted for timezone differences twice. We had `Timezone = 'UTC'` and `Time_format = 'isoUtcDatetime'` which resulted in the `Time` column displaying UTC-1 with UTC timestamps and a UTC+1 browser.