Improve documentation on ordered bulk operations for 2.6

The documentation on the new initializeOrderedBulkOp command needs some further detail on how the bulk operations are actually processed. In particular, "ordered" operations have a performance impact depending on how the operations are interleaved (consecutive operations that are the same can be be more effectively batched).

A few blog posts go into more helpful detail:

• http://christiankvalheim.com/post/the_new_bulk_api/
• http://blog.mongohq.com/better-bulking-for-mongodb-2-6-and-beyond/.