move comment on StringHeader to fix godoc output.
move comment on StringHeader to fix godoc output.

R=rsc, ken2
CC=golang-dev
http://codereview.appspot.com/196055

apparentlymart/golang,bdwalton/debian-golang,bdwalton/debian-golang,ssrl/go,ssrl/go,ssrl/go,gnanderson/go,bdwalton/debian-golang,ssrl/go,h8liu/golang,bdwalton/debian-golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tianon/debian-golang,nightlyone/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,bdwalton/debian-golang,pombredanne/go-deleteme,apparentlymart/golang,tianon/debian-golang,nightlyone/golang,pombredanne/go-deleteme,h8liu/golang,tv42/old-go,gnanderson/go,h8liu/golang,dsriram/golang,apparentlymart/golang,tianon/debian-golang,bdwalton/debian-golang,nightlyone/golang,bdwalton/debian-golang,h8liu/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,nightlyone/golang,dsriram/golang,dsriram/golang,nightlyone/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,ssrl/go,dsriram/golang,ssrl/go,tv42/old-go,tianon/debian-golang,tianon/debian-golang,gnanderson/go,tv42/old-go,tianon/debian-golang,h8liu/golang,bdwalton/debian-golang,gnanderson/go,tianon/debian-golang,h8liu/golang,nightlyone/golang,tv42/old-go,tianon/debian-golang,nightlyone/golang,gnanderson/go,dsriram/golang,gnanderson/go,h8liu/golang