Add documentation to is_node

graphene/relay/node.py

@@ -9,6 +9,12 @@ from ..types.interface import InterfaceMeta
 
 
 def is_node(objecttype):
+    '''
+    Check if the given objecttype has Node as an interface
+    '''
+    assert issubclass(objecttype, ObjectType), (
+        'Only ObjectTypes can have a Node interface.'
+    )
     for i in objecttype._meta.interfaces:
         if issubclass(i, Node):
             return True