Add a comment explaining what the check_repo job does

.github/workflows/latest_deps.yml

@@ -23,6 +23,10 @@ concurrency:
 
 jobs:
   check_repo:
+    # Prevent this workflow from running on any fork of Synapse other than matrix-org/synapse, as it is
+    # only useful to the Synapse core team.
+    # All other workflow steps depend on this one, thus if 'should_run_workflow' is not 'true', the rest
+    # of the workflow will be skipped as well.
     runs-on: ubuntu-latest
     outputs:
       should_run_workflow: ${{ steps.check_condition.outputs.should_run_workflow }}

.github/workflows/twisted_trunk.yml

@@ -19,6 +19,10 @@ concurrency:
 
 jobs:
   check_repo:
+    # Prevent this workflow from running on any fork of Synapse other than matrix-org/synapse, as it is
+    # only useful to the Synapse core team.
+    # All other workflow steps depend on this one, thus if 'should_run_workflow' is not 'true', the rest
+    # of the workflow will be skipped as well.
     if: github.repository == 'matrix-org/synapse'
     runs-on: ubuntu-latest
     outputs: