Add manage-namespaces documentation

https://github.com/rancher/webhook/blob/release/v0.4/docs.md#project-annotation

docs/reference-guides/rancher-webhook.md

@@ -126,6 +126,8 @@ The webhook provides extra validations on [namespaces](https://github.com/ranche
 - A user account with the proper permissions can pre-create the namespace with the appropriate configuration.
   - This option depends on the ability of the application to handle existing resources.
 
+Another one of these validations ensures that the user has the proper permissions to update the `field.cattle.io/projectId` annotation on a namespace. These permissions would be `manage-namespaces` for `projects` in `management.cattle.io`
+
 ## Issues on Specific Versions
 
 **Note:** The following is an incomplete list of high-severity issues affecting specific Rancher/webhook versions. In most cases, these issues can be resolved by upgrading to a more recent Rancher version.