It supports a single M.2 SATA storage device. It does not support an M.2 PCIe storage device. When used, the M.2 slot will disable the SATA_3 slot.
I seems as though it DOES support booting from an M.2 PCIe device if a PCIe slot adapter is used, but you will likely want to confirm that with ASRock.
Without an adapter, unless you are just trying to reduce cabling and save space, there isn't a lot to be gained by using it's M.2 SATA slot over using a regular SATA 2.5" drive. Speeds will be the same.