os: make ReadAt comment clearer
os: make ReadAt comment clearer

Fixes issue 2486.

R=golang-dev, hanwen
CC=golang-dev
http://codereview.appspot.com/5417064

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