[Android] Improve the description of XWalkView

We should notice users to call onDestroy() method of XWalkView
explicitly, or it will cause the memory leak due to the native
stuff of XWalk is not released, such as the WebGL conext.

BUG=XWALK-1778
And more, we can add more comment to specify that the `onDestroy` is equivalent  to  `destroy()` of `WebView`:
http://developer.android.com/reference/android/webkit/WebView.html#destroy()