Clarify Project import/export docs re imports to older GitLab versions

doc/user/project/settings/import_export.md

@@ -25,6 +25,8 @@ To set up a project import/export:
 
 Note the following:
 
+- Imports from a newer version of GitLab are not supported.
+  The Importing GitLab version must be greater than or equal to the Exporting GitLab version.
 - Imports will fail unless the import and export GitLab instances are
   compatible as described in the [Version history](#version-history).
 - Exports are stored in a temporary [shared directory](../../../development/shared_files.md)