Fix generated Retry documentation

The docs for generated Retry methods is:

// Retry enables retryable writes for this operation. Retries are not handled automatically,
// instead a boolean is returned from Execute and SelectAndExecute that indicates if the
// operation can be retried. Retrying is handled by calling RetryExecute.

This is incorrect and from a previous design of the Core API. Fix this to be accurate and up to date documentation.