Bug 29062: (QA follow-up) Add comment

Wrapped it in a TODO ;)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

C4/Circulation.pm

@@ -3475,6 +3475,8 @@ sub SendCirculationAlert {
         branches    => $branch,
     };
 
+    # TODO: Currently, we need to pass an issue_id as identifier for old_issues, but still an itemnumber for issues.
+    # See C4::Letters:: _parseletter_sth
     if( $type eq 'CHECKIN' ){
         $tables->{old_issues} = $issue->issue_id;
     } else {