add tofu comment

src/taler_config.ml

@@ -168,6 +168,7 @@ module type Offline_signing = sig
    - we need two different file here
    - there is three, not two, crypto helper modules
      is it only two, because the eddsa one is not comptabilized as a "crypto helper" here? *)
+  (* tofu = Trust On First Use *)
   val secm_tofu_file : file_path
   val secm_denom_pubkey : string option
   val secm_esign_pubkey : string option