Add note about using OpenShift API endpoints

Change-Id: I5f449416c7fea99249fe25043f147eb899c26b98

doc/source/using.rst

@@ -295,6 +295,13 @@ the image first::
 With curl
 ~~~~~~~~~
 
+.. note:: The following examples are querying the OpenShift API directly, not
+          the docker-registry server.
+          For more information about OpenShift API endpoints and how you can
+          use them, you can refer to the official OpenShift documentation_.
+
+.. _documentation: https://docs.openshift.com/container-platform/3.5/rest_api/openshift_v1.html
+
 For ``imagestreams``::
 
     curl https://registry.rdoproject.org:8443/oapi/v1/namespaces/tripleo/imagestreams/centos-binary-aodh-api
@@ -658,4 +665,4 @@ For ``imagestreamtags``::
         ],
         "dockerImageManifestMediaType": "application/vnd.docker.distribution.manifest.v1+json"
       }
-    }
+    }