Clarify deallocation semantics: NSOperationQueue vs @completion block

[stanislaw]

CompositeOperations uses its own completion block which is run after NSOperation is finished (KVO about isFinished is sent). When working with NSOperationQueue this can lead to earlier deallocation of operation when we are in completion block:

operation.completion = ^(id result, id error) {
    // so far it is always fired but there is no strict policy about this known to the author
}