Update documentation of --vmwarevsphere-hostsystem when using DRS-enabled clusters (docker#3572)

Author: ajfeatherstone

machine/drivers/vsphere.md

@@ -24,7 +24,7 @@ Creates machines on a [VMware vSphere](http://www.vmware.com/products/vsphere) V
 -   `--vmwarevsphere-datastore`: Datastore for Docker VM.
 -   `--vmwarevsphere-datacenter`: Datacenter for Docker VM (must be set to `ha-datacenter` when connecting to a single host).
 -   `--vmwarevsphere-pool`: Resource pool for Docker VM.
--   `--vmwarevsphere-hostsystem`: vSphere compute resource where the docker VM will be instantiated (use <cluster>/* or <cluster>/<host> if using a cluster).
+-   `--vmwarevsphere-hostsystem`: vSphere compute resource where the docker VM will be instantiated. This can be omitted if using a cluster with DRS.
 
 The VMware vSphere driver uses the latest boot2docker image.
 
@@ -44,4 +44,4 @@ The VMware vSphere driver uses the latest boot2docker image.
 | `--vmwarevsphere-datastore`       | `VSPHERE_DATASTORE`       | -                        |
 | `--vmwarevsphere-datacenter`      | `VSPHERE_DATACENTER`      | -                        |
 | `--vmwarevsphere-pool`            | `VSPHERE_POOL`            | -                        |
-| `--vmwarevsphere-hostsystem`      | `VSPHERE_HOSTSYSTEM`      | -                        |
+| `--vmwarevsphere-hostsystem`      | `VSPHERE_HOSTSYSTEM`      | -                        |