update the document

website/docs/r/virtual_network_gateway.html.markdown

@@ -104,11 +104,12 @@ EOF
 
 The following arguments are supported:
 
-* `ip_configuration` - (Required) One or more (up to 3) `ip_configuration` blocks documented below.
+* `ip_configuration` - (Required) One or more (up to 3) `ip_configuration` blocks documented below. Removing or overwrite existing `ip_configuration` forces a new resource to be created.
   An active-standby gateway requires exactly one `ip_configuration` block,
   an active-active gateway requires exactly two `ip_configuration` blocks whereas
   an active-active zone redundant gateway with P2S configuration requires exactly three `ip_configuration` blocks.
 
+
 * `location` - (Required) The location/region where the Virtual Network Gateway is located. Changing this forces a new resource to be created.
 
 * `name` - (Required) The name of the Virtual Network Gateway. Changing this forces a new resource to be created.