index_manager.md document is now short and concise.

Change-Id: I9b6183ea2087f82ef800d75405ebe3f75364548a
Reviewed-on: http://review.couchbase.org/34418
Reviewed-by: John Liang <john.liang@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>