Document every exported symbol and how callers use the library.

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

update.go

@@ -14,33 +14,38 @@ type Action int
 const (
 	// ActionAdd inserts a record.
 	ActionAdd Action = iota
-	// ActionDelete removes a matching RR (name, type, and rdata).
+	// ActionDelete removes one record that matches owner, type, and rdata.
 	ActionDelete
 )
 
-// Change is a typed add or delete of one record.
+// Change is one add or delete of one record inside an Update batch.
 type Change struct {
 	Action Action
 	Record Record
 }
 
-// Add builds an insert change.
+// Add builds an insert change for rec.
 func Add(rec Record) Change {
 	return Change{Action: ActionAdd, Record: rec}
 }
 
-// Delete builds a delete-by-rr change (name, type, and rdata).
+// Delete builds a delete of one matching record (owner, type, and rdata).
 func Delete(rec Record) Change {
 	return Change{Action: ActionDelete, Record: rec}
 }
 
-// UpdateResult is the nameserver's response to an UPDATE.
+// UpdateResult is a successful UPDATE response.
+// Rcode is the nameserver rcode text, which is NOERROR on success.
 type UpdateResult struct {
 	Rcode string
 }
 
-// Update applies RFC 2136 changes. It tries UDP first and retries on TCP when
-// the UDP exchange fails or the response is truncated.
+// Update sends an RFC 2136 UPDATE for changes, signed with the session TSIG key.
+// It tries UDP, then TCP when the UDP exchange fails or the response is truncated.
+// No changes returns success and does not contact the nameserver.
+// It returns an error when ctx is done, a record cannot be encoded, the
+// exchange fails, the nameserver rcode is not NOERROR, or Action is neither
+// ActionAdd nor ActionDelete.
 func (c *Client) Update(ctx context.Context, changes ...Change) (*UpdateResult, error) {
 	if err := ctx.Err(); err != nil {
 		return nil, err