MB-11209: Document blob related stats

Change-Id: I3c262363202edc1a601b7d0f188f7fdde257bddf
Reviewed-on: http://review.couchbase.org/37604
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>