Add documentation to rootfs functions

Clarifies role of varies rootfs functions as well as their return values.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

rootfs/diff.go

@@ -3,23 +3,24 @@ package rootfs
 import (
 	"fmt"
 
-	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/snapshot"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"golang.org/x/net/context"
 )
 
+// MountDiffer computes the difference between two mounts and returns a
+// descriptor for the computed diff. The provided ref can be used to track
+// the content creation of the diff and media type is used to determine the
+// format of the created content.
 type MountDiffer interface {
 	DiffMounts(ctx context.Context, lower, upper []mount.Mount, media, ref string) (ocispec.Descriptor, error)
 }
 
-type DiffOptions struct {
-	MountDiffer
-	content.Store
-	snapshot.Snapshotter
-}
-
+// Diff creates a layer diff for the given snapshot identifier from the parent
+// of the snapshot. A content ref is provided to track the progress of the
+// content creation and the provided snapshotter and mount differ are used
+// for calculating the diff. The descriptor for the layer diff is returned.
 func Diff(ctx context.Context, snapshotID, contentRef string, sn snapshot.Snapshotter, md MountDiffer) (ocispec.Descriptor, error) {
 	info, err := sn.Stat(ctx, snapshotID)
 	if err != nil {