Update shardCollection.txt

Add note on distribution of hash-shard-key chunks to shardCollection command. This currently exists in http://docs.mongodb.org/manual/tutorial/shard-collection-with-a-hashed-shard-key/ but should live here as well. MongoDB users who are familiar with hashed shard keys are more likely to view this page than the tutorial.

Signed-off-by: kay <kay.kim@10gen.com>
trivial: single source hashed shard key during chunk migration note