Document fork guard in workflow jobs

Author: guettli

.github/workflows/test.yml

@@ -27,6 +27,7 @@ jobs:
   cloud:
     name: e2e k8s-${{ matrix.k3s }}
     runs-on: ubuntu-latest
+    # Fork pull requests cannot access the required secrets, so only repository branches may run this job.
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
 
     permissions:
@@ -157,6 +158,7 @@ jobs:
   robot:
     name: e2e robot
     runs-on: ubuntu-latest
+    # Fork pull requests cannot access the required secrets, so only repository branches may run this job.
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
 
     permissions: