sysext: Fix config file section docs and --root= (#40063)

diff --cc src/sysext/sysext.c
index 1d129f42716a293ebdbcac9a91f9eb9c20cacc66,d18f1389370082d0cde0b4423521da51e0519964..2f6bee58c46d5222118110f0f9ce89cc938b9b03
--- 1/src/sysext/sysext.c
--- 2/src/sysext/sysext.c
+++ b/src/sysext/sysext.c
@@@ -179,12 -184,12 +184,12 @@@ static int parse_config_file(ImageClas
          r = config_parse_standard_file_with_dropins_full(
                          arg_root,
                          config_file,
-                         image_class == IMAGE_SYSEXT ? "SysExt\0" : "ConfExt\0",
+                         sections,
                          config_item_table_lookup, items,
                          CONFIG_PARSE_WARN,
 -                        /* userdata = */ NULL,
 -                        /* ret_stats_by_path = */ NULL,
 -                        /* ret_dropin_files = */ NULL);
 +                        /* userdata= */ NULL,
 +                        /* ret_stats_by_path= */ NULL,
 +                        /* ret_dropin_files= */ NULL);
          if (r < 0)
                  return r;