MB-10252 Add the descriptions of memory-related stats to stats doc

Change-Id: Ib0039cc6526eed54c32fb87c8584f8ac7e82c620
Reviewed-on: http://review.couchbase.org/39108
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>