message: Fix documentation for age::cli_common::read_secret
  Closes str4d/rage#216.
diff: @@ -101,7 +101,10 @@ where
  /// Requests a secret from the user.
  ///
  /// If a `pinentry` binary is available on the system, it is used to request the secret.
  -/// If not, we fall back to requesting directly in the CLI via stdin.
  +/// If not, we fall back to requesting directly in the CLI via a TTY.
  +///
  +/// This API does not take the secret directly from stdin, because it is specifically
  +/// intended to take the secret from a human.
  ///
  /// # Parameters
  ///
  