pmem state: document that only .json files matter

The implementation already worked like that, it just wasn't documented
and thus it was unknown whether reusing the directory also for other
local state (like the upcoming extra volume mounts) is okay.

pkg/pmem-state/pmem-state.go

@@ -44,7 +44,9 @@ var _ StateManager = &fileState{}
 
 // NewFileState instantiates the file state manager with given directory
 // location. It ensures the provided directory exists.
-// Returns error, if fails to create the directory in case of not pre-existing.
+// State entries are mapped to files with the .json suffix in that directory and
+// vice versa. Other directory content is ignored, which makes it possible
+// to use the directory also for other state information.
 func NewFileState(directory string) (StateManager, error) {
 	if err := ensureLocation(directory); err != nil {
 		return nil, err