[storage migration] Include a note to mention storageBackend migration can be rolled back mid migration.

It would be good to add a note to the storageBackend migration documentation that a migration can be rolled-back midway if a user chooses to.

For example consider a 3 node cluster: [n0, n1, n2].

1. Change storageBackend from couchstore to magma.
2. Add node n3 and rebalance out n0.
3. The storage backend is magma on n3 and is couchstore on n1, n2.

The user can now change the storageBackend to couchstore again now and if they rebalance out n3 and rebalance in a new node, the storageBackend on all the nodes will be couchstore.