Set arbiterOnly to true on MongoDB arbiter node

docs/mongodb_guide.md

@@ -291,6 +291,11 @@ all:
 To add an arbiter to the replica set, define a `mongodb_arbiter` group. Arbiters participate in
 elections but hold no data.
 
+An existing data-bearing member cannot be converted to an arbiter by re-running the playbook.
+MongoDB does not allow `arbiterOnly` to change in a reconfig, so the run fails at "Create or
+reconfigure the replicaset". Remove the member with `rs.remove()`, delete its data directory,
+restart `mongod`, and re-add it with `rs.addArb()`, or use a host that is not yet in the replica set.
+
 ```yaml
 all:
   children:

roles/mongodb/tasks/configure-mongodb-replicaset.yml

@@ -117,7 +117,8 @@
 - name: Add the arbiter to the list of servers when there is one
   ansible.builtin.set_fact:
     mongodb_servers: >-
-      {{ mongodb_servers + [{'host': item + ':' + (mongodb_port | string), 'priority': 0}] }}
+      {{ mongodb_servers + [{'host': item + ':' + (mongodb_port | string),
+      'priority': 0, 'arbiterOnly': true}] }}
   loop: "{{ groups.mongodb_arbiter }}"
   when:
     - mongodb_is_primary_node