Improve documentation for grouping by time

@sanderson is already working on this.

Since Jan 10, 2020.

This is for InfluxDB 1.x. I was surprised recently to discover that when I do a query using a time range as a where criteria and group by 10 seconds, InfluxDB locks the time windows to 00,10,20,30,40,50 seconds on the clock. What I expected to happen was that it would use 10 second windows starting from the timestamp of the beginning timestamp in the where clause.

In other words, if I did a query with a where and group like: _where time >= '2020-01-02T01:02:01.000000Z' and time < '2020-01-02T01:07:01.000000Z' GROUP BY time(10000ms), "host" FILL(0)_ the time windows will NOT be 01:02:01-01:02:11, 01:02:11-01:02:21, etc but instead will be 01:02:00-01:02:10, 01:02:10-01:02:20, etc.

This caused a strange behavior in an export script that exported data in chunks of 5 minute windows due to memory constraints, but the last data set it retrieved from InfluxDB would consistently be all 0s (due to the fill) because the window it was looking over would be very small and there would be no data recorded in it.

The documentation about group by time is pretty good, but all of the examples use 00 seconds as the start time, so it would be good to add some extra documentation around this behavior.