As we all know Atmel always have errors in the datasheets, but this one I guess needs a warning.
In the pdf file of the AVR instructionset is very unclear of which (and directly wrong) instructions the different AVR has, but normally you can go the the datasheet and verify the instructions.
But if you follow the mega328PB datasheet it doesn't have the MOVW instruction!!! (I hope that's this just an datasheet error because someone found it funny to rearrange the instructions!! ).