Document purpose of terraform-yaml resources

See conversation on #911

ci/pipeline.yml

@@ -177,6 +177,9 @@ jobs:
             resource: src
             trigger: false
             passed: [set-self]
+          # Changes to the iaas state file trigger a build. This is not a step
+          # input because the state is accessed separately using a
+          # terraform_remote_state data source.
           - get: terraform-yaml
             resource: terraform-yaml-development
             trigger: true
@@ -859,6 +862,9 @@ jobs:
             resource: terraform-config
             trigger: true
             passed: [terraform-apply-development]
+          # Changes to the iaas state file trigger a build. This is not a step
+          # input because the state is accessed separately using a
+          # terraform_remote_state data source.
           - get: terraform-yaml
             resource: terraform-yaml-staging
             trigger: true
@@ -1437,6 +1443,9 @@ jobs:
             resource: terraform-config
             passed: [acceptance-tests-staging]
             trigger: true
+          # Changes to the iaas state file trigger a build. This is not a step
+          # input because the state is accessed separately using a
+          # terraform_remote_state data source.
           - get: terraform-yaml
             resource: terraform-yaml-production
             trigger: true