[Update] Clarify gapfill for other grouping columns

[k-rus]

If a query contains more grouping columns than just time_bucket_gapfill the gaps on the other grouping values will be filled. It is good to explain how the values are filled and to give an example.

Example, query:

SELECT time_bucket_gapfill('3 hours', time, '2017-01-01 06:00', '2017-01-02 18:00'),
       device,
       first(value, time),
       avg(value)
FROM disttable
GROUP BY 1,2;

[solugebefola]

Following up on this issue. Is it still outstanding?