Respond to feedback.

- Fixed typos.
- Improved description in externs.
- Added audio to default media playlist codecs.
- Renamed some demo strings.

lib/util/player_configuration.js

@@ -30,7 +30,10 @@ goog.require('shaka.util.Platform');
  * @export
  */
 shaka.util.PlayerConfiguration = class {
-  /** @return {shaka.extern.PlayerConfiguration} */
+  /**
+   * @return {shaka.extern.PlayerConfiguration}
+   * @export
+   */
   static createDefault() {
     // This is a relatively safe default in the absence of clues from the
     // browser.  For slower connections, the default estimate may be too high.