archive/zip: tweak API and docs.

Fixes #2835.

R=golang-dev, n13m3y3r, r, bradfitz
CC=golang-dev
https://golang.org/cl/5645051

src/pkg/archive/zip/reader.go

@@ -117,7 +117,7 @@ func (rc *ReadCloser) Close() error {
 }
 
 // Open returns a ReadCloser that provides access to the File's contents.
-// It is safe to Open and Read from files concurrently.
+// Multiple files may be read concurrently.
 func (f *File) Open() (rc io.ReadCloser, err error) {
 	bodyOffset, err := f.findBodyOffset()
 	if err != nil {

src/pkg/archive/zip/writer.go

@@ -19,7 +19,7 @@ import (
 
 // Writer implements a zip file writer.
 type Writer struct {
-	*countWriter
+	countWriter
 	dir    []*header
 	last   *fileWriter
 	closed bool
@@ -32,7 +32,7 @@ type header struct {
 
 // NewWriter returns a new Writer writing a zip file to w.
 func NewWriter(w io.Writer) *Writer {
-	return &Writer{countWriter: &countWriter{w: bufio.NewWriter(w)}}
+	return &Writer{countWriter: countWriter{w: bufio.NewWriter(w)}}
 }
 
 // Close finishes writing the zip file by writing the central directory.