Add docs for accessing details of partially-completed imports

What does this MR do?

To support gitlab-com/www-gitlab-com!132803 (merged), and for #437874 (closed), add some documentation for the feature.

CC @justin_ho

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doc/user/group/import/index.md

@@ -264,11 +264,13 @@ To view group import history:
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/429109) in GitLab 16.6 [with a flag](../../feature_flags.md) named `bulk_import_details_page`. Enabled by default.
 > - Feature flag `bulk_import_details_page` removed in GitLab 16.8.
+> - Details for partially completed and completed imports [added](https://gitlab.com/gitlab-org/gitlab/-/issues/437874) in GitLab 16.9.
 
 To review the results of an import:
 
 1. Go to the [Group import history page](#group-import-history).
 1. To see the details of a failed import, select the **See failures** link on any import with a **Failed** or **Partially completed** status.
+1. If the import has a **Partially completed** or **Complete** status, to see which items were and were not imported, select **Details**.
 
 ## Migrated group items