io: clarify that ReadAt shouldn't move the seek offset
io: clarify that ReadAt shouldn't move the seek offset

R=r, mkrautz, r2, rsc
CC=golang-dev
http://codereview.appspot.com/4415041

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