document what the int returned by Exec represents

internal/db/read_writer.go

@@ -103,8 +103,9 @@ type Writer interface {
 	// DB returns the sql.DB
 	DB() (*sql.DB, error)
 
-	// Exec will execute the sql with the values as parameters. No options are
-	// currently supported
+	// Exec will execute the sql with the values as parameters. The int returned
+	// is the number of rows affected by the sql. No options are currently
+	// supported.
 	Exec(sql string, values []interface{}, opt ...Option) (int, error)
 
 	// GetTicket returns an oplog ticket for the aggregate root of "i" which can
@@ -191,8 +192,9 @@ func (rw *Db) DB() (*sql.DB, error) {
 	return rw.underlying.DB(), nil
 }
 
-// Exec will execute the sql with the values as parameters. No options are
-// currently supported.
+// Exec will execute the sql with the values as parameters. The int returned
+// is the number of rows affected by the sql. No options are currently
+// supported.
 func (rw *Db) Exec(sql string, values []interface{}, opt ...Option) (int, error) {
 	if sql == "" {
 		return NoRowsAffected, fmt.Errorf("missing sql: %w", ErrInvalidParameter)