Document breaking change in HcsFormatWritableLayerVhd

Add a comment on our wrapper of HcsFormatWritableLayerVhd to describe that
it expects a different handle on anything above ws2019. Additionally add
a comment above SetupBaseOSVolume stating what build it's supported on.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

computestorage/format.go

@@ -13,6 +13,11 @@ import (
 // FormatWritableLayerVhd formats a virtual disk for use as a writable container layer.
 //
 // If the VHD is not mounted it will be temporarily mounted.
+//
+// NOTE: This API had a breaking change in the operating system after Windows Server 2019.
+// On ws2019 the API expects to get passed a file handle from CreateFile for the vhd that
+// the caller wants to format. On > ws2019, its expected that the caller passes a vhd handle
+// that can be obtained from the virtdisk APIs.
 func FormatWritableLayerVhd(ctx context.Context, vhdHandle windows.Handle) (err error) {
 	title := "hcsshim::FormatWritableLayerVhd"
 	ctx, span := oc.StartSpan(ctx, title) //nolint:ineffassign,staticcheck

computestorage/setup.go

@@ -50,6 +50,9 @@ func SetupBaseOSLayer(ctx context.Context, layerPath string, vhdHandle windows.H
 // `volumePath` is the path to the volume to be used for setup.
 //
 // `options` are the options applied while processing the layer.
+//
+// NOTE: This API is only available on builds of Windows greater than 19645. The client
+// will need to handle if this is safe to call.
 func SetupBaseOSVolume(ctx context.Context, layerPath, volumePath string, options OsLayerOptions) (err error) {
 	if osversion.Build() < 19645 {
 		return errors.New("SetupBaseOSVolume is not present on builds older than 19645")