docs: explain deployment.environment impact on service identity

# Why

To clarify a likely misconception about service identity in the case
of multiple environments.

# What

Explain that `deployment.environment` does not impact service identity.

# References

Fixes #119

model/resource/deployment_environment.yaml

@@ -10,4 +10,12 @@ groups:
         brief: >
           Name of the [deployment environment](https://wikipedia.org/wiki/Deployment_environment)
           (aka deployment tier).
+        note: >
+          `deployment.environment` does not affect the uniqueness constraints defined through
+          the `service.namespace`, `service.name` and `service.instance.id` resource attributes.
+          This implies that resources carrying the following attribute combinations MUST be
+          considered to be identifying the same service:
+          
+           - `service.name=frontend`, `deployment.environment=production`
+           - `service.name=frontend`, `deployment.environment=staging`.
         examples: ['staging', 'production']