MB-100: added more documentation for babysitting VM feature

Change-Id: Iba1acae37128eb059f7733beba73057b493e476c
Reviewed-on: http://review.couchbase.org/26509
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>