DOCS-2079 and DOCS-2211: correct secondary throttle explanation

Signed-off-by: kay <kay.kim@10gen.com>
DOCS-2079: minor clarifications

Signed-off-by: kay <kay.kim@10gen.com>
DOCS-2079: fixed write concern on secondary throttle from 'majority' to '2'

Signed-off-by: kay <kay.kim@10gen.com>
DOCS-2079: reorg based on conversation between Sam and Zola

Signed-off-by: kay <kay.kim@10gen.com>
DOCS-2079 DOCS-2211 chunk migration and replication