testing/iotest: rephrase DataErrReader's description

R=r
CC=golang-dev
https://golang.org/cl/8650044

src/pkg/testing/iotest/reader.go

@@ -37,9 +37,11 @@ func (r *halfReader) Read(p []byte) (int, error) {
 	return r.r.Read(p[0 : (len(p)+1)/2])
 }
 
-// DataErrReader returns a Reader that returns the final
-// error with the last data read, instead of by itself with
-// zero bytes of data.
+// DataErrReader changes the way errors are handled by a Reader. Normally, a
+// Reader returns an error (typically EOF) from the first Read call after the
+// last piece of data is read. DataErrReader wraps a Reader and changes its
+// behavior so the final error is returned along with the final data, instead
+// of in the first call after the final data.
 func DataErrReader(r io.Reader) io.Reader { return &dataErrReader{r, nil, make([]byte, 1024)} }
 
 type dataErrReader struct {