Document the package, exported constants, and remaining helpers.

godoc now covers ActionAdd/ActionDelete, internal helpers, and UDP-then-TCP UPDATE behaviour.

Co-authored-by: Cursor <cursoragent@cursor.com>

update.go

@@ -12,7 +12,9 @@ import (
 type Action int
 
 const (
+	// ActionAdd inserts a record.
 	ActionAdd Action = iota
+	// ActionDelete removes a matching RR (name, type, and rdata).
 	ActionDelete
 )
 
@@ -38,7 +40,7 @@ type UpdateResult struct {
 }
 
 // Update applies RFC 2136 changes. It tries UDP first and retries on TCP when
-// the response is truncated.
+// the UDP exchange fails or the response is truncated.
 func (c *Client) Update(ctx context.Context, changes ...Change) (*UpdateResult, error) {
 	if err := ctx.Err(); err != nil {
 		return nil, err
@@ -72,6 +74,7 @@ func (c *Client) Update(ctx context.Context, changes ...Change) (*UpdateResult,
 	return &UpdateResult{Rcode: dns.RcodeToString[resp.Rcode]}, nil
 }
 
+// updateMessage builds a TSIG-signed RFC 2136 UPDATE message.
 func (c *Client) updateMessage(changes []Change) (*dns.Msg, error) {
 	zone := dns.Fqdn(c.zone)
 	msg := new(dns.Msg)