[#302] pkg/object: Document default values set in `NewSplitID`

Document field values of instance constructed via
`NewSplitID`.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/object/splitid.go

@@ -10,6 +10,9 @@ type SplitID struct {
 }
 
 // NewSplitID returns UUID representation of splitID attribute.
+//
+// Defaults:
+//  - id: random UUID.
 func NewSplitID() *SplitID {
 	return &SplitID{
 		uuid: uuid.New(),