Add more instructions about security wrt Riak Search, MapReduce

* Be explicit that security will break old Riak Search
* Add information about module-level security with Erlang MapReduce
* Be even louder about the fact that security, in general, must be enabled very carefully

Addresses #1219 

/cc @jonmeredith 
"will still be stored for security to be re-enabled" is a bit awkward. I would suggest: "Users, groups, and other security attributes will be stored in Riak, which means that you can re-enable security in the future using your previous configuration." Or something along those lines.