add description & update doc

Author: Pearl1594

cloudstack/resource_cloudstack_instance.go

@@ -188,6 +188,7 @@ func resourceCloudStackInstance() *schema.Resource {
 				Optional:     true,
 				ValidateFunc: validation.StringInSlice([]string{"Secure", "Legacy"}, true),
 				ForceNew:     true,
+				Description:  "The boot mode of the instance. Can only be specified when uefi is true. Valid options are 'Legacy' and 'Secure'.",
 			},
 
 			"start_vm": {

website/docs/r/instance.html.markdown

@@ -104,6 +104,8 @@ The following arguments are supported:
 
 * `uefi` - (Optional) When set, will boot the instance in UEFI/Legacy mode (defaults false)
 
+* `boot_mode` - (Optional) The boot mode of the instance. Can only be specified when uefi is true. Valid options are 'Legacy' and 'Secure'.
+
 ## Attributes Reference
 
 The following attributes are exported: