Document situation where the PRX tx queue can stall

[thalesfragoso]

Since this implementation uses a FIFO, the tx queue (in PRX mode) will stall if the first item to be popped doesn't match the pipe address of the received packet. The packets will still be acked with a zero sized payload, but the queue will be stalled until the device receives a packet that matches the pipe address of the first item in the queue.

We should also provide a method to allow the users to pop a packet from the queue in case they need to deal with this situation.