Add comment for changed line

plugins/virtuser_file/virtuser_file.php

@@ -68,6 +68,8 @@ class virtuser_file extends rcube_plugin
             $arr = preg_split('/\s+/', trim($r[$i]));
 
             if (count($arr) > 0) {
+                // Replace '\@' with '@' to handle cases where internal usernames include an '@' character.
+                // Sometimes usernames with '@' are saved with a leading '\' to avoid conflicts.
                 $p['user'] = trim(str_replace('\@', '@', $arr[count($arr) - 1]));
                 break;
             }