Confirm Project forking behaviour via the API

* It's unclear that the forking operation can still be ongoing
* `import_status` is available in Project Entity to determine status

changelogs/unreleased/28607-forking-and-configuring-project-via-api-works-very-unreliable.yml

+---
+title: Confirm Project forking behaviour via the API
+merge_request:
+author:

doc/api/projects.md

@@ -551,6 +551,8 @@ Parameters:
 Forks a project into the user namespace of the authenticated user or the one provided.
+The forking operation for a project is asynchronous and is completed in a background job. The request will return immediately. To determine whether the fork of the project has completed, query the `import_status` for the new project.
+
 ```
 POST /projects/:id/fork
 ```