Extend docs about UTC (#4094)
Extend docs about UTC (#4094)

* Extend docs about UTC

* Update timeunit.md

vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite