AUFS storage driver reference should mention system requirements / compatibility

[thaJeztah]

File: engine/userguide/storagedriver/aufs-driver.md

Noticed this during the discussion on moby/moby#29873, the AUFS document in the storage driver section does not mention prerequisites, so users trying to configure their daemon to use the aufs storage driver, will still get an error "driver not supported", unless those prerequisites are in place. Only place this is mentioned is in the installation documentation for ubuntu; https://docs.docker.com/engine/installation/linux/ubuntulinux/#/prerequisites-by-ubuntu-version

[docker-robott]

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