fmt: add sentence about return values to docs for Printf etc.
fmt: add sentence about return values to docs for Printf etc.

R=rsc, gri
CC=golang-dev
http://codereview.appspot.com/1952045

ssrl/go,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,ssrl/go,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,h8liu/golang,ssrl/go,ssrl/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,h8liu/golang,h8liu/golang,dsriram/golang,gnanderson/go,h8liu/golang,apparentlymart/golang,pombredanne/go-deleteme,ssrl/go,ssrl/go,tv42/old-go,dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,h8liu/golang,gnanderson/go,apparentlymart/golang,h8liu/golang,h8liu/golang,ssrl/go