Merge pull request #49077 from ym1024/update_rbac_md

Add descriptions of ClusterRole and ClusterRoleBinding to docs/reference/glossary/rbac.md

Author: k8s-ci-robot

content/en/docs/reference/glossary/rbac.md

@@ -15,5 +15,18 @@ tags:
 
 <!--more--> 
 
-RBAC utilizes *roles*, which contain permission rules, and *role bindings*, which grant the permissions defined in a role to a set of users.
+RBAC utilizes four kinds of Kubernetes objects:
 
+Role
+: Defines permission rules in a specific namespace.
+
+ClusterRole
+: Defines permission rules cluster-wide.
+
+RoleBinding
+: Grants the permissions defined in a role to a set of users in a specific namespace.
+
+ClusterRoleBinding
+: Grants the permissions defined in a role to a set of users cluster-wide.
+
+For more information, see [RBAC](/docs/reference/access-authn-authz/rbac/).