Full Reach integration with APM is currently a work in progress.
Right now, you can use Reach for drone positioning by configuring solution output. For example, you can connect Reach to Navio via UART and configure solution output to UART in NMEA format. You will, however, need to handle base correction delivery separately. Our goal is to make base corrections available through the ground control station, but that requires testing. Stay tuned.
We are sorry about the missing info in the Docs, they are being updated regularly.