Instance resource docs state instance metadata update constraints

Author: ccushing

website/docs/r/core_instance.html.markdown

@@ -177,7 +177,9 @@ The following arguments are supported:
 
 	curl http://169.254.169.254/opc/v1/instance/ curl http://169.254.169.254/opc/v1/instance/metadata/ curl http://169.254.169.254/opc/v1/instance/metadata/<any-key-name>
 
-	You'll get back a response that includes all the instance information; only the metadata information; or the metadata information for the specified key name, respectively. 
+	You'll get back a response that includes all the instance information; only the metadata information; or the metadata information for the specified key name, respectively.
+	
+	**Note:** Both the 'user_data' and 'ssh_authorized_keys' fields cannot be changed after an instance has launched. Any request which updates, removes, or adds either of these fields will be rejected. You must provide the same values for 'user_data' and 'ssh_authorized_keys' that already exist on the instance. 
 * `preserve_boot_volume` - (Optional) Specifies whether to delete or preserve the boot volume when terminating an instance. The default value is false. Note: This value only applies to destroy operations initiated by Terraform.
 * `shape` - (Required) The shape of an instance. The shape determines the number of CPUs, amount of memory, and other resources allocated to the instance.