I've never had a letter without this. It also doesn't buy a lawsuit. Generally, your whole employment is "at will." You can be fired at any time.
Including discretionary bonus information is not only standard but expected. It shows how the company treats and values its employees. It sets out expectations for both parties.
Frankly, if I had to ask about a standard level of communication about my compensation package, I'd be questioning if I wanted to work at that company.