Fix some discrepancies with max_open_files

This PR is based on an email from Chris Boyton. He mentioned there were a few issues with our explanation of the `max_open_files` defaults. After talking to @matthewvon, in 1.4.x:

* The default is 30
* The minimum Riak will accept is 20
* If you've got nothing listed, we'll make it 1000 

This attempts to clear up some of the confusion. A sanity check from @matthewvon and/or @evanmcc would be great.