MB-100: Add some documentation about internals of the vtree

Change-Id: I6b99b256e95bf603d3be940f4c8fcb3ea5c57a6a
Reviewed-on: http://review.couchbase.org/21026
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>