Add some comments in the query code.
Add some comments in the query code.

No code changes.  This is only improving internal doc for the code
handling "group-by" style queries.

Change-Id: I31bc6ab4d9c1ff8a9b8b997349c2abe7ce848f45

OpenTSDB/opentsdb,OpenTSDB/opentsdb,andyflury/opentsdb,MadDogTechnology/opentsdb,kidaa/opentsdb,OpenTSDB/opentsdb,eswdd/opentsdb,ShefronYudy/opentsdb,johann8384/opentsdb,alienth/opentsdb,alienth/opentsdb,eswdd/opentsdb,ShefronYudy/opentsdb,sidhhu/opentsdb,johann8384/opentsdb,MadDogTechnology/opentsdb,sidhhu/opentsdb,MadDogTechnology/opentsdb,andyflury/opentsdb,kidaa/opentsdb