docs: reflect prefix location optionality in upgrading guide

docs/upgrading_to_v2.0.md

@@ -4,6 +4,7 @@ The v2.0 release of the terraform-google-cloud-storage module is a backwards inc
 
 - The module now uses for_each instead of count.
 - The minimum Terraform version was increased to 0.13.
+- The bucket doesn't have location as mandatory part of the name prefix.
 
 ## Migration Instructions
 
@@ -36,6 +37,13 @@ terraform state mv module.cloud_storage.google_storage_bucket.buckets[0] 'module
 terraform state mv module.cloud_storage.google_storage_bucket.buckets[1] 'module.cloud_storage.google_storage_bucket.buckets["two"]'
 ```
 
+Bucket prefix contained `location` before, so to preserve the same bucket name with `location` in prefix you should make some adjustments:
+
+```diff
+- prefix = var.project_id 
++ prefix ="${var.project_id}-${var.location}"
+```
+
 Re-running the plan should show that the storage bucket resources are no longer showing a diff.
 
 ```