[vsphere] update install-config description for diskType

This bug was initially created as a copy of
Bug #2096605
I am copying this bug because: the parent bug solved the validation aspect of diskType but now the description of diskType in
https://github.com/openshift/installer/blob/master/data/data/install.openshift.io_installconfigs.yaml#L2914-L2923
needs to be updated.

Version: 4.11.0-0.nightly-2022-06-06-201913

Platform: vSphere IPI

What happened?
1. If user inputs an invalid value for platform.vsphere.diskType in install-config.yaml file, there is no validation checking for diskType and doesn't exit with error, but continues the installation, which is not the same behavior as in 4.10.

After all vms are provisioned, I checked that the disk provision type is thick.

2. If user doesn't set platform.vsphere.diskType in install-config.yaml file, the default disk provision type is thick, but not the vSphere default storage policy. On VMC, the default policy is thin, so maybe the description of diskType should also need to be updated.

$ ./openshift-install explain installconfig.platform.vsphere.diskType
KIND: InstallConfig
VERSION: v1

RESOURCE: <string>
Valid Values: "","thin","thick","eagerZeroedThick"
DiskType is the name of the disk provisioning type, valid values are thin, thick, and eagerZeroedThick. When not specified, it will be set according to the default storage policy of vsphere.

What did you expect to happen?
validation for diskType

How to reproduce it (as minimally and precisely as possible)?
set diskType to invalid value in install-config.yaml and install the cluster