io: amend ReaderFrom doc as per r's comment

R=r
CC=golang-dev
https://golang.org/cl/6458097

src/pkg/io/io.go

@@ -131,9 +131,9 @@ type ReadWriteSeeker interface {
 
 // ReaderFrom is the interface that wraps the ReadFrom method.
 //
-// ReadFrom reads data from r until EOF. The return value n is the
-// number of bytes read. Any error except io.EOF encountered during
-// the read is also returned.
+// ReadFrom reads data from r until EOF or error.
+// The return value n is the number of bytes read.
+// Any error except io.EOF encountered during the read is also returned.
 //
 // The Copy function uses ReaderFrom if available.
 type ReaderFrom interface {