compress/gzip: document Reader.Read

Fixes #17374.

Change-Id: Ic89c35aaa31f35a8a4e3ffa09f49b68f08127625
Reviewed-on: https://go-review.googlesource.com/30718
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/compress/gzip/gunzip.go

@@ -238,6 +238,7 @@ func (z *Reader) readHeader() (hdr Header, err error) {
 	return hdr, nil
 }
 
+// Read implements io.Reader, reading uncompressed bytes from its underlying Reader.
 func (z *Reader) Read(p []byte) (n int, err error) {
 	if z.err != nil {
 		return 0, z.err