os: make ReadAt comment clearer
Fixes #2486.
R=golang-dev, hanwen
CC=golang-dev
https://golang.org/cl/5417064
diff --git a/src/pkg/os/file.go b/src/pkg/os/file.go
index 386afb8..6a8d346 100644
--- a/src/pkg/os/file.go
+++ b/src/pkg/os/file.go
@@ -70,8 +70,8 @@
// ReadAt reads len(b) bytes from the File starting at byte offset off.
// It returns the number of bytes read and the error, if any.
-// EOF is signaled by a zero count with err set to io.EOF.
-// ReadAt always returns a non-nil error when n != len(b).
+// ReadAt always returns a non-nil error when n < len(b).
+// At end of file, that error is io.EOF.
func (file *File) ReadAt(b []byte, off int64) (n int, err error) {
if file == nil {
return 0, EINVAL
