Gson customization docs

Hi! I noticed that https://github.com/square/retrofit/pull/286 added docs for customizing the Gson behavior but had unaddressed comments (and was a few months old). I've made the edits suggested by @JakeWharton and rebased off master.

I built on the original work by @austynmahoney with a few small changes to highlight that these customizations are not part of Retrofit, but rather that Retrofit is just delegating to Gson.

Let me know if anything else needs to happen before this gets merged.

Cheers!