goprotobuf: Include package docs from .proto in the generated package docs.
goprotobuf: Include package docs from .proto in the generated package docs.

These kinds of comments in .proto files usually aren't there
but if they are then they are valuable to have in the generated code.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/65860043

go-mirrors/goprotobuf,VividCortex/goprotobuf