message: Fix documentation of put_missing
diff: @@ -81,8 +81,8 @@ config_value& put(settings& dict, string_view key, T&& value) {
  return put_impl(dict, key, tmp);
  }
  -/// Converts `value` to a `config_value` and assigns it to `key` if `value` is
  -/// currently missing in `xs`.
  +/// Converts `value` to a `config_value` and assigns it to `key` unless `xs`
  +/// already contains `key` (does nothing in this case).
  /// @param xs Dictionary of key-value pairs.
  /// @param key Human-readable nested keys in the form `category.key`.
  /// @param value New value for given `key`.
  