Update split.txt wrt splitting empty chunks

This used to say that `split` doesn't support splitting empty chunks, but elsewhere (later in this document and in http://docs.mongodb.org/manual/tutorial/create-chunks-in-sharded-cluster/) we recommend using `split` to pre-split an empty sharded collection.

Signed-off-by: kay <kay.kim@10gen.com>
tweak split command clarification