Document the `environment` attribute. (#13360)

The existence of the attribute is mentioned here already: https://www.terraform.io/docs/state/environments.html

website/source/docs/providers/terraform/d/remote_state.html.md

@@ -31,6 +31,7 @@ resource "aws_instance" "foo" {
 The following arguments are supported:
 
 * `backend` - (Required) The remote backend to use.
+* `environment` - (Optional) The Terraform environment to use.
 * `config` - (Optional) The configuration of the remote backend.
  * Remote state config docs can be found [here](/docs/backends/types/atlas.html)