Refine documentation for enable_auto_anti_affinity_drs_rules property

See related cloudfoundry#378

Extracted from https://www.pivotaltracker.com/n/projects/1133984/stories/133642741
 >  We can create a DRS rule based on env.bosh.group (skip if this field is not provided).

jobs/vsphere_cpi/spec

@@ -24,7 +24,7 @@ properties:
   vcenter.datacenters:
     description: Datacenters in vCenter to use (value is an array of Hashes representing datacenters and clusters, See director.yml.erb.erb)
   vcenter.enable_auto_anti_affinity_drs_rules:
-    description: Creates DRS rule to place VMs on separate hosts.
+    description: Creates anti-affinity DRS rules for each instance group of each bosh deployment to place VMs on separate hosts. The DRS rules are named from template: <bosh-director-name>-<bosh-deployment-name>-<instance-group-name>
     default: false
   vcenter.memory_reservation_locked_to_max:
     description: When enabled, the requested memory for the VMs will be reserved in vCenter. Enabling this will likely lead to wasted memory on the hosts and will prevent vCenter from making the best use of the memory resources.