Returns the block to execute when the operation’s main task is complete.
Operation objects monitor the isFinished key path and execute this block when the value at that key path changes to YES. As a result, this block is called regardless of whether the operation completed successfully or was cancelled.
Original signature is '- (void (^)(void))completionBlock'


- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)