[DOCS] Clarify variable usage in global_overrides
[DOCS] Clarify variable usage in global_overrides

Change-Id: I2a69e16376e01493e65738d7a41903b54f5fb3a5
Closes-bug: #1596572

Logan2211/openstack-ansible,weezer/openstack-ansible,openstack/openstack-ansible,bunchc/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,cloudnull/os-ansible-deployment,BjoernT/openstack-ansible,BjoernT/os-ansible-deployment,robb-romans/openstack-ansible,cloudnull/os-ansible-deployment,yanyao/openstack-ansible,weezer/openstack-ansible,BjoernT/os-ansible-deployment,os-cloud/os-ansible-deployment,yanyao/newton,cfarquhar/openstack-ansible,yanyao/openstack-ansible,cfarquhar/openstack-ansible,Logan2211/openstack-ansible,stackforge/os-ansible-deployment,os-cloud/os-ansible-deployment,yanyao/newton,BjoernT/os-ansible-deployment,BjoernT/openstack-ansible,robb-romans/openstack-ansible,bunchc/openstack-ansible