Incorrect documentation on handleRebalanceEvent

[fgm]

The end of the method comment appears to be wrong. It says:

// In the polling case (not channel based consumer) the rebalance event
// is returned in retval, else nil is returned.

While the code actually returns a non-nil event only in the channel case, which is the exact opposite of the comment.

• lines 1022-1025: creates ev if there is a rebalanceCb or appRebalanceEnable
• lines 1029-1034 : returns it if channel != nil && c.appRebalanceEnable && c.rebalanceCb == nil which is the opposite of the documentation
• line 1046: return nil if there is a rebalanceCb and it called Assign/Unassign
• line 1088: return nil unconditionally

It seems that:

• either the doc should read like this instead:

   // In the polling case (not channel based consumer) the rebalance event
   // is passed to the `rebalanceCb` if it is set, and never returned to `eventPoll`
  

• or the function logic should be amended to match the comment and allow the method to return the event to eventPoll.

The latter would provide most consistent handling, but would make it easier for callers to introduce bugs in their code because then the protection logic built when calling rebalanceCb would have no equivalent for the processing in eventPoll.

Do you want a PR either way ?

(tangentially related to #624 / #625)