Add warning to inventory about zuul gerrit server config

Let's avoid changing this and breaking Depends-On again by adding an
explicit warning to the code that sets the config.

Change-Id: Idcb77d8b0b53c56ea7f15f18e001f8bc9a001c98

inventory/service/group_vars/zuul.yaml

@@ -18,6 +18,10 @@ zuul_connections:
 
   - name: 'gerrit'
     driver: 'gerrit'
+    # Changing this value impacts the functionality of Depends-On in Zuul.
+    # Zuul expects Depends-On URLs to contain this server value in them.
+    # If they do not then they aren't considered as valid Depends-On and
+    # are ignored.
     server: 'review.opendev.org'
     canonical_hostname: 'opendev.org'
     user: 'zuul'