add description & update doc

cloudstack/resource_cloudstack_instance.go

diff --git a/cloudstack/resource_cloudstack_instance.go b/cloudstack/resource_cloudstack_instance.go
index b4cc318..4caa345 100644
--- a/cloudstack/resource_cloudstack_instance.go
+++ b/cloudstack/resource_cloudstack_instance.go
@@ -188,6 +188,7 @@
 				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

diff --git a/website/docs/r/instance.html.markdown b/website/docs/r/instance.html.markdown
index ee3aef4..7b0b1bd 100644
--- a/website/docs/r/instance.html.markdown
+++ b/website/docs/r/instance.html.markdown
@@ -104,6 +104,8 @@
 
 * `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: