Two short paragraphs is about all you need, as you note. Keeping it objective serves you and it serves the company. And as part of the exit process, it helps to know that anything said (or over-said) in such communication is rarely likely to change anything. As long as you share that you're leaving, when you're leaving, and that you plan to make the transition seamless, that's all you should trouble yourself with including. Pleasantries and memories can be kept to the exit conversation.