fmt: tweak the doc for %U. Fixes issue 1727.
fmt: tweak the doc for %U.
Fixes issue 1727.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4437069

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