Improve singleton provider docs

docs/providers/singleton.rst

@@ -32,6 +32,13 @@ factories:
 - :ref:`factory-specialize-provided-type`
 - :ref:`abstract-factory`
 
+``Singleton`` provider scope is tied to the container. Two different containers will provider
+two different singleton objects:
+
+.. literalinclude:: ../../examples/providers/singleton_multiple_containers.py
+   :language: python
+   :lines: 3-
+
 Resetting memorized object
 --------------------------
 

examples/providers/singleton_multiple_containers.py

@@ -0,0 +1,24 @@
+"""`Singleton` provider resetting example."""
+
+from dependency_injector import containers, providers
+
+
+class UserService:
+    ...
+
+
+class Container(containers.DeclarativeContainer):
+
+    user_service_provider = providers.Singleton(UserService)
+
+
+if __name__ == '__main__':
+    container1 = Container()
+    user_service1 = container1.user_service_provider()
+    assert user_service1 is container1.user_service_provider()
+
+    container2 = Container()
+    user_service2 = container2.user_service_provider()
+    assert user_service2 is container2.user_service_provider()
+
+    assert user_service1 is not user_service2

examples/providers/singleton_resetting.py

@@ -19,5 +19,5 @@ if __name__ == '__main__':
 
     container.user_service_provider.reset()
 
-    users_service2 = container.user_service_provider()
+    user_service2 = container.user_service_provider()
-    assert users_service2 is not user_service1
+    assert user_service2 is not user_service1