Disabling OS disk encryption is supported

Disabling OS disk encryption is supported on Windows

```
$ az vm get-instance-view --resource-group winade --name winade --query instanceView.osName
"Windows Server 2016 Datacenter"

$ az vm extension show --resource-group winade --vm-name winade --name AzureDiskEncryption --query "settings.{EncryptionOperation:EncryptionOperation, VolumeType:VolumeType}"
{
  "EncryptionOperation": "EnableEncryption",
  "VolumeType": "All"
}

$ az vm encryption show --resource-group winade --name winade --query "disks[].{name:name, status:statuses}"
[
  {
    "name": "winade_OsDisk_1_40aad40920b64d6ba4fa37571edfe876",
    "status": [
      {
        "code": "EncryptionState/encrypted",
        "displayStatus": "Encryption is enabled on disk",
        "level": "Info",
        "message": null,
        "time": null
      }
    ]
  },
  {
    "name": "a",
    "status": [
      {
        "code": "EncryptionState/encrypted",
        "displayStatus": "Encryption is enabled on disk",
        "level": "Info",
        "message": null,
        "time": null
      }
    ]
  }
]

$ az vm encryption disable --name winade --resource-group winade --volume-type all

$ az vm encryption show --resource-group winade --name winade --query "disks[].{name:name, status:statuses}"
[
  {
    "name": "winade_OsDisk_1_40aad40920b64d6ba4fa37571edfe876",
    "status": [
      {
        "code": "EncryptionState/notEncrypted",
        "displayStatus": "Disk is not encrypted",
        "level": "Info",
        "message": null,
        "time": null
      }
    ]
  },
  {
    "name": "a",
    "status": [
      {
        "code": "EncryptionState/notEncrypted",
        "displayStatus": "Disk is not encrypted",
        "level": "Info",
        "message": null,
        "time": null
      }
    ]
  }
]
```

Author: taoyama

articles/virtual-machines/windows/disk-encryption-faq.md

@@ -44,8 +44,6 @@ The [Azure Disk Encryption overview](disk-encryption-overview.md) article lists
 
 You can encrypt both boot and data volumes, but you can't encrypt the data without first encrypting the OS volume.
 
-After you've encrypted the OS volume, disabling encryption on the OS volume isn't supported.
-
 ## Can I encrypt an unmounted volume with Azure Disk Encryption?
 
 No, Azure Disk Encryption only encrypts mounted volumes.