Add a lot of documentation

copy_done.go

@@ -7,8 +7,11 @@ import (
 type CopyDone struct {
 }
 
+// Backend identifies this message as sendable by the PostgreSQL backend.
 func (*CopyDone) Backend() {}
 
+// Decode decodes src into dst. src must contain the complete message with the exception of the initial 1 byte message
+// type identifier and 4 byte message length.
 func (dst *CopyDone) Decode(src []byte) error {
 	if len(src) != 0 {
 		return &invalidMessageLenErr{messageType: "CopyDone", expectedLen: 0, actualLen: len(src)}
@@ -17,10 +20,12 @@ func (dst *CopyDone) Decode(src []byte) error {
 	return nil
 }
 
+// Encode encodes src into dst. dst will include the 1 byte message type identifier and the 4 byte message length.
 func (src *CopyDone) Encode(dst []byte) []byte {
 	return append(dst, 'c', 0, 0, 0, 4)
 }
 
+// MarshalJSON implements encoding/json.Marshaler.
 func (src *CopyDone) MarshalJSON() ([]byte, error) {
 	return json.Marshal(struct {
 		Type string