Document the go structs used by clients when they import CSI Proxy

[mauriciopoppe]

Document files like https://github.com/kubernetes-csi/csi-proxy/blob/master/pkg/{api-group}/types.go e.g. https://github.com/kubernetes-csi/csi-proxy/blob/master/pkg/filesystem/types.go

[mauriciopoppe]

Solved in #266