unicode/utf8: document return value for decode errors Also replace archaic definition of rune.
unicode/utf8: document return value for decode errors
Also replace archaic definition of rune.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5654048

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go