Revisit max object size recommendation in Riak

On the FAQ [we advise](http://docs.basho.com/riak/latest/cookbooks/faqs/developing-faq/#is-there-a-limit-on-the-file-size-that-can-be-stor) to not store anything larger than 50MB. 

We contradict ourselves on the [2i config page](http://docs.basho.com/riak/latest/cookbooks/Secondary-Indexes---Configuration) and recommend 4-6MB. 

We need to get our story straight. :)