code: function updateUserDataField(userId, data, overwrite, callback) {
          return _updateUserObjectField(userId, ['data'], data, overwrite)
              .then(function (userData) {
                  return userData.data;
              })
              .nodeify(callback);
      }
docs: Updates the provided fields in data (recursively) within userData.data.
  @param {string} userId
  @param {object} data
  @param {boolean} [overwrite]  - if true the settings for the key will be overwritten.
  @param {function} [callback]
  @returns {*}