Update docs regarding Popper (not a callback function argument anymore)

Please verify these documentation changes to make sure my assessment is correct.

Looking over the code, I found that a `Popper` has to be called to exhaustion (ie.. until it returns `Nothing`) before feeding more data or flushing.  Failure to do so may result in data corruption (due to discarded input) and segfaults (due to `finishDeflate` and `flushDeflate` not touching the ByteString that keeps the `next_in` bytes alive).