add notes to View register JS and CSS file methods

explain why assetmanager features are not available in these methods.

close #14382
related to #10297, #9101

framework/web/View.php

@@ -386,6 +386,11 @@ public function registerCss($css, $options = [], $key = null)
 
     /**
      * Registers a CSS file.
+     *
+     * This method should be used for simple registration of CSS files. If you want to use features of
+     * [[AssetManager]] like appending timestamps to the URL and file publishing options, use [[AssetBundle]]
+     * and [[registerAssetBundle()]] instead.
+     *
      * @param string $url the CSS file to be registered.
      * @param array $options the HTML attributes for the link tag. Please refer to [[Html::cssFile()]] for
      * the supported options. The following options are specially handled and are not treated as HTML attributes:
@@ -446,6 +451,11 @@ public function registerJs($js, $position = self::POS_READY, $key = null)
 
     /**
      * Registers a JS file.
+     *
+     * This method should be used for simple registration of JS files. If you want to use features of
+     * [[AssetManager]] like appending timestamps to the URL and file publishing options, use [[AssetBundle]]
+     * and [[registerAssetBundle()]] instead.
+     *
      * @param string $url the JS file to be registered.
      * @param array $options the HTML attributes for the script tag. The following options are specially handled
      * and are not treated as HTML attributes: