Revert/disable the slash syntax for kubectl explain group/version

pkg/kubectl/cmd/explain.go

@@ -72,17 +72,18 @@ func RunExplain(f *cmdutil.Factory, out io.Writer, cmd *cobra.Command, args []st
 	apiV := cmdutil.GetFlagString(cmd, "api-version")
 
 	mapper, _ := f.Object()
-	group, inModel, fieldsPath, err := kubectl.SplitAndParseResourceRequest(args[0], mapper)
+	// TODO: After we figured out the new syntax to separate group and resource, allow
+	// the users to use it in explain (kubectl explain <group><syntax><resource>).
+	// Refer to issue #16039 for why we do this. Refer to PR #15808 that used "/" syntax.
+	inModel, fieldsPath, err := kubectl.SplitAndParseResourceRequest(args[0], mapper)
 	if err != nil {
 		return err
 	}
 
-	if len(group) == 0 {
-		// TODO: We should deduce the group for a resource by discovering the supported resources at server.
-		group, err = mapper.GroupForResource(inModel)
-		if err != nil {
-			return err
-		}
+	// TODO: We should deduce the group for a resource by discovering the supported resources at server.
+	group, err := mapper.GroupForResource(inModel)
+	if err != nil {
+		return err
 	}
 
 	if len(apiV) == 0 {

pkg/kubectl/explain.go

@@ -41,11 +41,10 @@ func GetSwaggerSchema(apiVer string, kubeClient client.Interface) (*swagger.ApiD
 }
 
 // SplitAndParseResourceRequest separates the users input into a model and fields
-func SplitAndParseResourceRequest(inResource string, mapper meta.RESTMapper) (string, string, []string, error) {
+func SplitAndParseResourceRequest(inResource string, mapper meta.RESTMapper) (string, []string, error) {
 	inResource, fieldsPath := splitDotNotation(inResource)
-	group, inResource := splitGroupFromResource(inResource)
 	inResource, _ = mapper.ResourceSingularizer(expandResourceShortcut(inResource))
-	return group, inResource, fieldsPath, nil
+	return inResource, fieldsPath, nil
 }
 
 // PrintModelDescription prints the description of a specific model or dot path
@@ -86,15 +85,6 @@ func PrintModelDescription(inModel string, fieldsPath []string, w io.Writer, swa
 	return printModelInfo(w, pointedModel, pointedModelAsProp)
 }
 
-func splitGroupFromResource(resource string) (string, string) {
-	seg := strings.SplitN(resource, "/", 2)
-	if len(seg) == 1 {
-		return "", seg[0]
-	} else {
-		return seg[0], seg[1]
-	}
-}
-
 func splitDotNotation(model string) (string, []string) {
 	var fieldsPath []string
 	dotModel := strings.Split(model, ".")