Update Downloadable Fonts docs to better explain the available fonts

https://twitter.com/gwanaxymatt/status/1035701272085577730 says,

> Google Fonts for Android developer docs say everything from http://fonts.google.com is supported. But Korean and Japanese fonts don't work via the API, and quite a few Arabic and Hebrew fonts seem to be missing as well. Any ideas? (cited doc: https://developers.google.com/fonts/docs/android)
>
> More specifically, the list of fonts returned by the Developer API (https://developers.google.com/fonts/docs/developer_api) doesn't match the fonts available via Google Fonts for Android. Are these updated at different times? If so, where to get the current list for Android?