docs: add godoc comments to all exported functions and methods

mc/proto/replay/reader.go

@@ -16,6 +16,7 @@ type Reader struct {
 	Version int32
 }
 
+// NewReader creates a new replay Reader from an io.Reader.
 func NewReader(r io.Reader) *Reader {
 	return &Reader{r: r}
 }

mc/proto/replay/writer.go

@@ -13,6 +13,7 @@ type Writer struct {
 	w io.Writer
 }
 
+// NewWriter creates a new replay Writer from an io.Writer.
 func NewWriter(w io.Writer) *Writer {
 	return &Writer{w: w}
 }