website: Update googlecompute engine docs

This change updates the documentation to describe how `network` and
`subnetwork` properties are processed.

website/source/docs/builders/googlecompute.html.md

@@ -216,7 +216,10 @@ builder.
     instance.
 
 -   `network` (string) - The Google Compute network id or URL to use for the
-    launched instance. Defaults to `"default"`.
+    launched instance. Defaults to `"default"`. If the value is not a URL, it
+    will be interpolated to `projects/((network_project_id))/global/networks/((network))`.
+    This value is not required if a `subnet` is specified.
+
 
 -   `network_project_id` (string) - The project ID for the network and subnetwork
     to use for launched instance. Defaults to `project_id`.
@@ -259,7 +262,9 @@ builder.
 -   `subnetwork` (string) - The Google Compute subnetwork id or URL to use for
     the launched instance. Only required if the `network` has been created with
     custom subnetting. Note, the region of the subnetwork must match the `region`
-    or `zone` in which the VM is launched.
+    or `zone` in which the VM is launched. If the value is not a URL, it
+    will be interpolated to `projects/((network_project_id))/regions/((region))/subnetworks/((subnetwork))`
+
 
 -   `tags` (array of strings)