samples: net: google_iot: Fix unclear documentation

The instructions on setting the necessary Kconfig values is unclear.
Add a description based on values that can be found in the Google IoT
Core console.

Signed-off-by: David Brown <[email protected]>

samples/net/cloud/google_iot_mqtt/README.rst

@@ -35,12 +35,28 @@ Run the ``create_keys.py`` script in the
 ``samples/net/cloud/google_iot_mqtt/src/private_info/`` directory.
 
 Users will also be required to configure the following Kconfig options
-based on their Google Cloud IOT project:
-
-- CLOUD_CLIENT_ID - created from <telemetry>/<region>/registries/<registry name>/devices/<device name>
-- CLOUD_AUDIENCE - created from telemetry without projects on front
-- CLOUD_SUBSCRIBE_CONFIG - created from /devices/<device name>/config
-- CLOUD_PUBLISH_TOPIC - created from /devices/<device name>/state
+based on their Google Cloud IOT project.  The following values come
+from the Google Cloud Platform itself:
+
+- PROJECT_ID: When you select your project at the top of the UI, it
+  should have a "name", and there should be an ID field as well.  This
+  seems to be two words and a number, separated by hyphens.
+- REGION: The Region shows in the list of registries for your
+  registry.  And example is "us-central1".
+- REGISTRY_ID: Each registry has an id.  This is a string given when
+  creating the registry.
+- DEVICE_ID: A name given for each device.  When viewing the table of
+  devices, this will be shown.
+
+From these values, the config values can be set using the following
+template:
+
+.. code-block: kconfig
+
+   CLOUD_CLIENT_ID="projects/PROJECT_ID/locations/REGION/registries/REGISTRY_ID/devices/DEVICE_ID"
+   CLOUD_AUDIENCE="PROJECT_ID"
+   CLOUD_SUBSCRIBE_CONFIG="/devices/DEVICE_ID/config"
+   CLOUD_PUBLISH_TOPIC="/devices/DEVICE_ID/state"
 
 See `Google Cloud MQTT Documentation
 <https://cloud.google.com/iot/docs/how-tos/mqtt-bridge>`_.