[ci skip] document fields option Model.validate

Closes #6714 Fixes #6703

lib/model.js

@@ -3513,6 +3513,7 @@ class Model {
   *
   * @param {Object} [options] Options that are passed to the validator
   * @param {Array} [options.skip] An array of strings. All properties that are in this array will not be validated
+  * @param {Array} [options.fields] An array of strings. Only the properties that are in this array will be validated
   * @param {Boolean} [options.hooks=true] Run before and after validate hooks
   *
   * @return {Promise<undefined|Errors.ValidationError>}