go/doc: add "hdr-" prefix to headers generated from package overviews.
go/doc: add "hdr-" prefix to headers generated from package overviews.

R=golang-dev, bradfitz, gri
CC=golang-dev
https://codereview.appspot.com/6935071

gnanderson/go,gnanderson/go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,gnanderson/go