If you are using Dgraph binaries. It will write to the same folder that is using the binaries. If you made a setup via script or other than Docker. There will be a default folder. Dgraph will use the system path where it was installed.
There is no recommendation for non-Docker systems for this exactly. But you can create your own solution. Can raise an NFS server and use it nicely with symlink.
Storage is a hardware-to-system default. In the case of applications we usually speak of “Volumes”. But there is a recommendation as to Storage indeed. If you want to get the most out of Dgraph it is recommended to use SSD’s of the best you have. It is not an explicit recommendation, but it exists.