Add some doc to the github actions / CI scripts (#8034)

* Add some doc to the githuib actions / CI scripts

Type: task

* alalytics-events is more name matching

.github/workflows/test_coverage.yml

@@ -7,6 +7,7 @@ jobs:
   test-coverage:
     runs-on: ubuntu-latest
     env:
+      # This must be set for fetchdep.sh to get the right branch
       PR_NUMBER: ${{github.event.number}}
     steps:
       - name: Checkout code