Group access tokens cannot import projects

Import process always attempts at adding the importing user as a
direct member to the target project or group. This does not work
when the importing user is a group (or project) access token bot
account because of restrictions that prevent such a user from being
a direct member of any group other than the top level one.

This change adds a troubleshooting entry with the error
that is observed when a group access token is used to attempt
a project import.

doc/user/project/settings/import_export.md

@@ -383,3 +383,17 @@ s = Gitlab::ImportExport::Saver.new(exportable: p, shared:p.import_export_shared
 s.send(:compress_and_save)
 s.send(:save_upload)
 ```
+
+### Import using the REST API fails when using a group access token
+
+[Group access tokens](../../group/settings/group_access_tokens.md)
+don't work for project or group import operations. When a group access token initiates an import,
+the import fails with this message:
+
+```plaintext
+Error adding importer user to Project members.
+Validation failed: User project bots cannot be added to other groups / projects
+```
+
+To use [Import REST APIs](../../../api/project_import_export.md),
+pass regular user account credentials such as [personal access tokens](../../profile/personal_access_tokens.md).