Update documentation for google_compute_region_per_instance_config (#4791) (#3261)

* docs: fix region defention for region stateful mig

`google_compute_instance_group_manager` is not defined anywhere in the
example and here we are talking about region instance groups and not
zonal.

* docs: add required update_policy for stateful migs

When you don't specify an `update_policy` and run `terraform apply` you
end up getting the following error:

```
Error waiting to create RegionPerInstanceConfig: Error waiting for Creating RegionPerInstanceConfig: CreateInstances can be used only when instance redistribution is disabled (set to NONE).
```

Also specify
[`minimal_action`](https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_region_instance_group_manager#minimal_action)
and
[`type`](https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_region_instance_group_manager#type)
since they are both required for `update_policy`

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/4791.txt

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

website/docs/r/compute_region_per_instance_config.html.markdown

@@ -72,6 +72,12 @@ resource "google_compute_region_instance_group_manager" "rigm" {
     instance_template = google_compute_instance_template.igm-basic.self_link
   }
 
+  update_policy {
+    type                         = "OPPORTUNISTIC"
+    instance_redistribution_type = "NONE"
+    minimal_action               = "RESTART"
+  }
+
   base_instance_name = "rigm"
   region             = "us-central1"
   target_size        = 2
@@ -86,7 +92,7 @@ resource "google_compute_disk" "default" {
 }
 
 resource "google_compute_region_per_instance_config" "with_disk" {
-  region = google_compute_instance_group_manager.igm.region
+  region = google_compute_region_instance_group_manager.igm.region
   region_instance_group_manager = google_compute_region_instance_group_manager.rigm.name
   name = "instance-1"
   preserved_state {