The manufacturer website is where you usually want to go to find the documentation. The product datasheet will specify the peripherals and register set for the chip you're looking at, including a description of each register and the fields within them. Some companies have a different name for this document. E.g. ST usually define their registers in a Reference Manual.
Here's the datasheet for the ATMega328: http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-42735-...