goprotobuf: Doc updates for extensions.
goprotobuf: Doc updates for extensions.

R=r
CC=golang-dev
http://codereview.appspot.com/6495093

wfxiang08/goprotobuf,liumuqi/goprotobuf,t3rm1n4l/goprotobuf,supiyun/goprotobuf,polydawn-ports/goprotobuf,tonnerre/go-protobuf,adwinsky/goprotobuf,ruiaylin/goprotobuf