Clarify need for default_bucket_props property in Riak app.config as of 1.4

Clarify that the "{default_bucket_props, [{allow_mult, true}]}," line under riak_core in the Riak app.config on http://docs.basho.com/riakcs/latest/cookbooks/configuration/Configuring-Riak/ is a requirement as of 1.4 and Riak CS will not start without it.  

Also provide explanation for why this is required.