docs(framework) Update client_fn docstrings to new signature

src/py/flwr/client/app.py

@@ -139,7 +139,7 @@ class `flwr.client.Client` (default: None)
     Starting an SSL-enabled gRPC client using system certificates:
 
     >>> def client_fn(context: Context):
-    >>>     return FlowerClient()
+    >>>     return FlowerClient().to_client()
     >>>
     >>> start_client(
     >>>     server_address=localhost:8080,

src/py/flwr/client/client_app.py

@@ -91,7 +91,7 @@ class ClientApp:
     >>> class FlowerClient(NumPyClient):
     >>>     # ...
     >>>
-    >>> def client_fn(node_id: int, partition_id: Optional[int]):
+    >>> def client_fn(context: Context):
     >>>    return FlowerClient().to_client()
     >>>
     >>> app = ClientApp(client_fn)

src/py/flwr/simulation/app.py

@@ -111,9 +111,9 @@ def start_simulation(
     Parameters
     ----------
     client_fn : ClientFnExt
-        A function creating Client instances. The function must have the signature
-        `client_fn(node_id: int, partition_id: Optional[int]). It should return
-        a single client instance of type Client. Note that the created client
+        A function creating `Client` instances. The function must have the signature
+        `client_fn(context: Context). It should return
+        a single client instance of type `Client`. Note that the created client
         instances are ephemeral and will often be destroyed after a single method
         invocation. Since client instances are not long-lived, they should not attempt
         to carry state over method invocations. Any state required by the instance