[js] Actions API changes

-  Update documentation
-  Resurrect the legacy API (too difficult to accurately translate touch actions
   in the new API to legacy commands). The legacy API will be removed once
   Chrome and Edge support the new API.

For #4564

Author: jleyba

javascript/node/selenium-webdriver/CHANGES.md

@@ -151,8 +151,15 @@ mode.
 
 *  Revamped the actions API to conform with the WebDriver Spec:
    <https://www.w3.org/TR/webdriver/#actions>. For details, refer to the JS doc
-   on the `lib/webdriver.ActionSequence` class. For simplicity, support for the
-   legacy actions API has been removed.
+   on the `lib/webdriver.ActionSequence` class.
+
+   As of December, 2017, only Firefox supports this new API.
+   `ActionSequence#perform()` will attempt to translate mouse and keyboard
+   sequences to the legacy API. Alternatively, you may continue to use the
+   legacy API directly via the `lib/actions` module. __NOTE:__ The legacy API
+   is considered strongly deprecated and will be removed in a minor release once
+   Google's Chrome and Microsoft's Edge browsers support the new API.
+
 *  All window manipulation commands are now supported.