Added descriptions to the various classes which were lacking one.

Change-Id: I88c1e1e93709917352beac88a065d84bfe574c2c
Reviewed-on: http://review.couchbase.org/29265
Tested-by: Brett Lawson <brett19@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>