btrfs: Add comment to Put()

Document why we don't need to do anything in Put().

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

graphdriver/btrfs/btrfs.go

@@ -206,6 +206,8 @@ func (d *Driver) Get(id string) (string, error) {
 }
 
 func (d *Driver) Put(id string) {
+	// Get() creates no runtime resources (like e.g. mounts)
+	// so this doesn't need to do anything.
 }
 
 func (d *Driver) Exists(id string) bool {