compress/gzip: clarify behavior of Writer.Close

Fixes #20551

Change-Id: Ia47cae14a26fe5f278ad7209218d083cc50a3ff8
Reviewed-on: https://go-review.googlesource.com/44572Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/compress/gzip/gzip.go

@@ -222,8 +222,9 @@ func (z *Writer) Flush() error {
 	return z.err
 }
 
-// Close closes the Writer, flushing any unwritten data to the underlying
-// io.Writer, but does not close the underlying io.Writer.
+// Close closes the Writer by flushing any unwritten data to the underlying
+// io.Writer and writing the GZIP footer.
+// It does not close the underlying io.Writer.
 func (z *Writer) Close() error {
 	if z.err != nil {
 		return z.err