Example (now deleted):
I'm going to say no. The point of a code review is to identify things about the code that can be improved and discuss possible improvements. It is not for the reviewers to explain the reviewed code to the person who came in with the code.
In the linked question the OP does not want to know what can be improved about the code nor does he want help improving it. He wants to understand what it does.
No, I think allowing them here would be a slippery slope. I think these questions would get answered on StackOverflow.
No. Pasting a code snippet and asking a question about it is a major SO activity. It belongs there for sure.