Fix out of date comments for CRI store packages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/image/image.go

@@ -161,7 +161,7 @@ func (s *Store) Resolve(ref string) (string, error) {
 
 // Get gets image metadata by image id. The id can be truncated.
 // Returns various validation errors if the image id is invalid.
-// Returns storeutil.ErrNotExist if the image doesn't exist.
+// Returns errdefs.ErrNotFound if the image doesn't exist.
 func (s *Store) Get(id string) (Image, error) {
 	return s.store.get(id)
 }