DOC: add ioutil.ProgressWriter comments

util/ioutil/progress.go

@@ -12,6 +12,7 @@ type ProgressWriter struct {
 	status chan int
 }
 
+// NewProgressWriter wraps io.Writer with total written size and a status channel.
 func NewProgressWriter(w io.Writer) *ProgressWriter {
 	return &ProgressWriter{
 		wr:     w,
@@ -30,30 +31,37 @@ func (pw *ProgressWriter) sum(n int) {
 	}
 }
 
+// Size returns total written size of io.Writer.
 func (pw *ProgressWriter) Size() int {
 	return pw.size
 }
 
+// Status returns the status channel.
+// Total written size will be sent to the status channel without blocking after every write operation.
 func (pw *ProgressWriter) Status() chan int {
 	if pw.status == nil {
 		pw.status = make(chan int)
 	}
 	return pw.status
 }
 
+// Close sends total written size to the blocking channel and then closes the channel.
+// Only the sender should close a channel, never the receiver.
 func (pw *ProgressWriter) Close() {
 	if pw.status != nil {
 		pw.status <- pw.size
 		close(pw.status)
 	}
 }
 
+// Write implements the standard io.Writer interface.
 func (pw *ProgressWriter) Write(p []byte) (n int, err error) {
 	n, err = pw.wr.Write(p)
 	pw.sum(n)
 	return n, err
 }
 
+// WriteString implements the standard io.StringWriter interface.
 func (pw *ProgressWriter) WriteString(s string) (n int, err error) {
 	if sw, ok := pw.wr.(io.StringWriter); ok {
 		n, err = sw.WriteString(s)