Fix out of date comments for CRI store packages

All of the CRI store related packages all use the standard errdefs
errors now for if a key doesn't or already exists (ErrAlreadyExists,
ErrNotFound), but the comments for the methods still referenced
some unused package specific error definitions. This change just
updates the comments to reflect what errors are actually returned
and adds comments for some previously undocumented exported functions.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

pkg/cri/store/snapshot/snapshot.go

@@ -56,7 +56,7 @@ func (s *Store) Add(snapshot Snapshot) {
 	s.snapshots[snapshot.Key] = snapshot
 }
 
-// Get returns the snapshot with specified key. Returns store.ErrNotExist if the
+// Get returns the snapshot with specified key. Returns errdefs.ErrNotFound if the
 // snapshot doesn't exist.
 func (s *Store) Get(key string) (Snapshot, error) {
 	s.lock.RLock()