Add the related documentation to the Transit Gateway route to support…

… blackhole

website/docs/r/ec2_transit_gateway_route.html.markdown

@@ -12,6 +12,8 @@ Manages an EC2 Transit Gateway Route.
 
 ## Example Usage
 
+### Standard usage
+
 ```hcl
 resource "aws_ec2_transit_gateway_route" "example" {
   destination_cidr_block         = "0.0.0.0/0"
@@ -20,12 +22,23 @@ resource "aws_ec2_transit_gateway_route" "example" {
 }
 ```
 
+### Blackhole route
+
+```hcl
+resource "aws_ec2_transit_gateway_route" "example" {
+  destination_cidr_block         = "0.0.0.0/0"
+  blackhole                      = true
+  transit_gateway_route_table_id = "${aws_ec2_transit_gateway.example.association_default_route_table_id}"
+}
+```
+
 ## Argument Reference
 
 The following arguments are supported:
 
 * `destination_cidr_block` - (Required) IPv4 CIDR range used for destination matches. Routing decisions are based on the most specific match.
-* `transit_gateway_attachment_id` - (Required) Identifier of EC2 Transit Gateway Attachment.
+* `transit_gateway_attachment_id` - (Optional) Identifier of EC2 Transit Gateway Attachment (required if `blackhole` is set to false).
+* `blackhole` - (Optional) Indicates whether to drop traffic that matches this route (default to `false`).
 * `transit_gateway_route_table_id` - (Required) Identifier of EC2 Transit Gateway Route Table.
 
 ## Attribute Reference