moveChunk and split commands have/will have new field parameters for 2.4

Required before 2.4.

moveChunk and split commands will have a new field added (think it'll be "bounds" in both cases, but still in-review) to allow better selection and splitting of chunks with hashed shard keys. We need to doc this new field (not sure if in a new 2.4 branch or everywhere with version noted?).

Problem with hashed shard keys is that it's non-trivial to figure out which chunk a key will hash to, so using "find" with a document to select a chunk for moving or splitting is not very useful.