Document how to override swap_device

Author: priteau

etc/kayobe/ansible/swap.yml

@@ -6,7 +6,9 @@
 # * swap_group: Host pattern against which to target the playbook. Default is
 #               'overcloud'.
 # * swap_device: Name of the swap device to configure. Default is
-#                '/dev/rootvg/lv_swap'.
+#                '/dev/rootvg/lv_swap'. To use a different value, set an extra
+#                var (e.g. in $KAYOBE_CONFIG_PATH/swap.yml) because play vars
+#                take precedence over any inventory variable.
 
 - name: Configure swap
   hosts: "{{ swap_group | default('overcloud') }}"