JCBC-386: Cleanup Query and add docblock for grouping.

This changeset performs non-behaviour-changing cleanup and
enhances the docblock for setGroup and setGroupLevel to
warn users to not use both together since they override
each other.

Change-Id: Icfb73a6c7c53c83e2957d62226c95638776ef399
Reviewed-on: http://review.couchbase.org/31250
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>