Update sharding-introduction.txt

source/core/sharding-introduction.txt

@@ -201,17 +201,16 @@ has 100 chunks on *shard 1* and 50 chunks on *shard 2*, the balancer
 will migrate chunks from *shard 1* to *shard 2* until the collection
 achieves balance.
 
-The shards manage *chunk migrations* as a background operation. During
-migration, all requests for a chunk's data address the "origin" shard.
-
-In a chunk migration, the *destination shard* receives all the
-documents in the chunk from the *origin shard*. Then, the destination
+The shards manage *chunk migrations* as a background operation between 
+an *origin shard* and a *destination shard*.  During a chunk migration,
+the *destination shard* is sent all the current documents in the chunk
+from the *origin shard*. Next, the destination
 shard captures and applies all changes made to the data during
-migration process. Finally, the destination shard updates the metadata
-regarding the location of the chunk on *config server*.
+the migration process. Finally, the metadata
+regarding the location of the chunk on *config server* is updated.
 
 If there's an error during the migration, the balancer aborts the
-process leaving the chunk on the origin shard. MongoDB removes the
+process leaving the chunk unchanged on the origin shard. MongoDB removes the
 chunk's data from the origin shard **after** the migration completes
 successfully.