Add an example of Composer 3 configuration with network attachment (#11288) (#18966)

[upstream:6a9d8c19ec6af03bbf6704ddce35aed5e3c21b3a]

Signed-off-by: Modular Magician <magic-modules@google.com>

Author: modular-magician

.changelog/11288.txt

@@ -0,0 +1,3 @@
+```release-note:none
+
+```

website/docs/r/composer_environment.html.markdown

@@ -25,12 +25,11 @@ To get more information about Environments, see:
   * [Connect an environment to a VPC network](https://cloud.google.com/composer/docs/composer-3/connect-vpc-network)
 * [Apache Airflow Documentation](http://airflow.apache.org/)
 
-<Note>
+-> **Note**
   Cloud Composer 1 is in the post-maintenance mode. Google does 
   not release any further updates to Cloud Composer 1, including new versions 
   of Airflow, bugfixes, and security updates. We recommend using
   Cloud Composer 2 or Cloud Composer 3 instead.
-</Note>
 
 Several special considerations apply to managing Cloud Composer environments 
 with Terraform:
@@ -92,18 +91,16 @@ resource "google_composer_environment" "test" {
 }
 ```
 
-
 ### With GKE and Compute Resource Dependencies
 
-<Note>
+-> **Note**
   To use custom service accounts, you must give at least the
   `role/composer.worker` role to the service account of the Cloud Composer 
   environment. For more information, see the
   [Access Control](https://cloud.google.com/composer/docs/how-to/access-control)
   page in the Cloud Composer documentation.
   You might need to assign additional roles depending on specific workflows 
   that the Airflow DAGs will be running.
-<Note>
 
 #### GKE and Compute Resource Dependencies (Cloud Composer 3)
 
@@ -302,6 +299,69 @@ resource "google_project_iam_member" "composer-worker" {
 }
 ```
 
+### Cloud Composer 3 networking configuration
+
+In Cloud Composer 3, networking configuration is simplified compared to
+previous versions. You don't need to specify network ranges, and can attach
+custom VPC networks to your environment.
+
+-> **Note**
+  It's not possible to detach a VPC network using Terraform. Instead, you can
+  attach a different VPC network in its place, or detach the network using
+  other tools like Google Cloud CLI.
+
+Use Private IP networking:
+
+```hcl
+resource "google_composer_environment" "example" {
+  name = "example-environment"
+  region = "us-central1"
+
+  config {
+
+    enable_private_ip_environment = true
+
+    # ... other configuration parameters
+  }
+}
+```
+
+Attach a custom VPC network (Cloud Composer creates a new network attachment):
+
+```hcl
+resource "google_composer_environment" "example" {
+  name = "example-environment"
+  region = "us-central1"
+
+  config {
+
+    node_config {
+      network = "projects/example-project/global/networks/example-network"
+      subnetwork = "projects/example-project/regions/us-central1/subnetworks/example-subnetwork"
+    }
+
+    # ... other configuration parameters
+  }
+}
+```
+
+Attach a custom VPC network (use existing network attachment):
+
+```hcl
+resource "google_composer_environment" "example" {
+  name = "example-environment"
+  region = "us-central1"
+
+  config {
+
+    node_config {
+      composer_network_attachment = projects/example-project/regions/us-central1/networkAttachments/example-network-attachment
+    }
+
+    # ... other configuration parameters
+  }
+}
+```
 
 ### With Software (Airflow) Config
 
@@ -327,6 +387,7 @@ resource "google_composer_environment" "test" {
   }
 }
 ```
+
 ## Argument Reference - Cloud Composer 1
 
 The following arguments are supported: