compress/{gzip,zlib}: mention that Close flushes

Our other CloseFlushers (archive/tar, compress/flate) do mention this.

The original change was accidentally submitted to the release branch:
        https://golang.org/cl/117430043/

TBR=rsc
R=r, rsc
CC=golang-codereviews
https://golang.org/cl/124130043

src/pkg/compress/gzip/gzip.go

@@ -245,7 +245,8 @@ func (z *Writer) Flush() error {
 	return z.err
 }
 
-// Close closes the Writer. It does not close the underlying io.Writer.
+// Close closes the Writer, flushing any unwritten data to the underlying
+// io.Writer, but does not close the underlying io.Writer.
 func (z *Writer) Close() error {
 	if z.err != nil {
 		return z.err

src/pkg/compress/zlib/writer.go

@@ -174,7 +174,8 @@ func (z *Writer) Flush() error {
 	return z.err
 }
 
-// Calling Close does not close the wrapped io.Writer originally passed to NewWriter.
+// Close closes the Writer, flushing any unwritten data to the underlying
+// io.Writer, but does not close the underlying io.Writer.
 func (z *Writer) Close() error {
 	if !z.wroteHeader {
 		z.err = z.writeHeader()