Customize: Support instantiation of partials with flat/unwrapped params for parity with controls, sections, and panels in [41726].

* Passing `options.params` when constructing `Partial` is now deprecated in favor of just passing `options`.
* Improve usage of jsdoc in JS `Partial` class.
* Also add `defaults` property to `wp.customize.selectiveRefresh.Partial` class for parity with `Control`.

See #42083.


git-svn-id: https://develop.svn.wordpress.org/trunk@42037 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/customize-controls.js

@@ -3401,6 +3401,12 @@
 	api.Control = api.Class.extend({
 		defaultActiveArguments: { duration: 'fast', completeCallback: $.noop },
 
+		/**
+		 * Default params.
+		 *
+		 * @since 4.9.0
+		 * @var {object}
+		 */
 		defaults: {
 			label: '',
 			description: '',