compress/gzip: clarify Latin-1 restrictions on gzip.Header

Fixes #12361.

Change-Id: Ifd62e8d93b2d733e67e0186c7185cd6291d3bbc1
Reviewed-on: https://go-review.googlesource.com/13939Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/gzip/gunzip.go

@@ -43,6 +43,9 @@ var (
 
 // The gzip file stores a header giving metadata about the compressed file.
 // That header is exposed as the fields of the Writer and Reader structs.
+//
+// Strings must be UTF-8 encoded and may only contain Unicode code points
+// U+0001 through U+00FF, due to limitations of the GZIP file format.
 type Header struct {
 	Comment string    // comment
 	Extra   []byte    // "extra data"

src/compress/gzip/gzip.go

@@ -44,10 +44,7 @@ type Writer struct {
 // Writes may be buffered and not flushed until Close.
 //
 // Callers that wish to set the fields in Writer.Header must do so before
-// the first call to Write or Close. The Comment and Name header fields are
-// UTF-8 strings in Go, but the underlying format requires NUL-terminated ISO
-// 8859-1 (Latin-1). NUL or non-Latin-1 runes in those strings will lead to an
-// error on Write.
+// the first call to Write, Flush, or Close.
 func NewWriter(w io.Writer) *Writer {
 	z, _ := NewWriterLevel(w, DefaultCompression)
 	return z