Docs: PS availability for project imports with author information retained

doc/user/project/settings/import_export.md

@@ -47,6 +47,8 @@ Note the following:
 - Imported users can be mapped by their primary email on self-managed instances, if an administrative user (not an owner) does the import.
   Otherwise, a supplementary comment is left to mention that the original author and
   the MRs, notes, or issues are owned by the importer.
+  - For project migration imports performed over GitLab.com Groups, preserving author information is
+    possible through a [professional services engagement](https://about.gitlab.com/services/migration/).
 - If an imported project contains merge requests originating from forks,
   then new branches associated with such merge requests are created
   within a project during the import/export. Thus, the number of branches