archive/tar: document Reader.Next's behavior at the end

Change-Id: I72f6d0fc66dbee3f832d2d960b99a166a5bb10c3
Reviewed-on: https://go-review.googlesource.com/2191Reviewed-by: David Symonds <dsymonds@golang.org>

src/archive/tar/reader.go

@@ -85,6 +85,8 @@ const (
 func NewReader(r io.Reader) *Reader { return &Reader{r: r} }
 
 // Next advances to the next entry in the tar archive.
+//
+// io.EOF is returned at the end of the input.
 func (tr *Reader) Next() (*Header, error) {
 	var hdr *Header
 	if tr.err == nil {