Comment on buffer/packet relocation

Change-Id: Ic19065b5c43fb7074fc261365b5930103f3b9243
Reviewed-on: http://review.couchbase.org/29610
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>