Trying to clarify when majority vs. w:2 is used

Maybe it should be restructured to be:

Points when chunk move waits for acknowledgement from majority of the replica set on the shard:

- Before starting the commit/writing chunk's new location to config database (waits for recipient shard to ack)
- After delete of documents on the "from" shard (waits for donor to ack)

Signed-off-by: kay <kay.kim@10gen.com>