doc: fix cln-grpc example.

- Add instruction to generate `primitives_pb2.py` file,
- Import `grpc` in the Python script.

Changelog-None

doc/developers-guide/app-development/grpc.md

@@ -54,13 +54,23 @@ python -m grpc_tools.protoc \
   --experimental_allow_proto3_optional
 ```
 
-
-
 This will generate two files in the current directory:
 
 - `node_pb2.py`: the description of the protobuf messages we'll be exchanging with the server.
 - `node_pb2_grpc.py`: the service and method stubs representing the server-side methods as local objects and associated methods.
 
+Finally, we generate the file `primitives_pb2.py` that contains
+protobuf messages imported in `node_pb2.py` file by running the
+following command:
+
+```bash
+python -m grpc_tools.protoc \
+  -I lightning/cln-grpc/proto \
+  path/to/cln-grpc/proto/primitives.proto \
+  --python_out=. \
+  --experimental_allow_proto3_optional
+```
+
 ### Connecting to the node
 
 Finally we can use the generated stubs and mTLS identity to connect to the node:
@@ -69,6 +79,7 @@ Finally we can use the generated stubs and mTLS identity to connect to the node:
 from pathlib import Path
 from node_pb2_grpc import NodeStub
 import node_pb2
+import grpc
 
 p = Path(".")
 cert_path = p / "client.pem"
@@ -82,7 +93,7 @@ creds = grpc.ssl_channel_credentials(
 )
 
 channel = grpc.secure_channel(
-	f"localhost:{grpc_port}",
+	"localhost:<GRPC-PORT>",
 	creds,
 	options=(('grpc.ssl_target_name_override', 'cln'),)
 )
@@ -91,7 +102,8 @@ stub = NodeStub(channel)
 print(stub.Getinfo(node_pb2.GetinfoRequest()))
 ```
 
-
+Note that we must replace `<GRPC-PORT>` by the corresponding port we
+used as `--grpc-port` option when we started our node.
 
 In this example, we first load the client identity as well as the CA certificate so we can verify the server's identity against it. We then create a `creds` instance using those details. Next we open a secure channel, i.e., a channel over TLS with verification of identities.