feat(storage): document storage policy configuration for disk settings

Signed-off-by: Pascal T. <pascal@toepke.dev>

Author: pascalinthecloud

docs-partials/builder/vsphere/common/DiskConfig-not-required.mdx

@@ -9,4 +9,8 @@
 - `disk_controller_index` (int) - The assigned disk controller for the disk.
   Defaults to the first controller, `(0)`.
 
+- `storage_policy` (string) - The name of the storage policy to apply to the disk. The storage policy
+  must already exist on the vCenter Server. If not specified, the default
+  storage policy of the target datastore is used.
+
 <!-- End of code generated from the comments of the DiskConfig struct in builder/vsphere/common/storage_config.go; -->