Ask HN: What are internal dev documentation best practices for a remote company?

My company has 3 developers, but we are about to have only 2. There are large sections that only the leaving developer has worked on. Are there any principles of developer documentation we should begin to follow in order to save time and money when an employee has to pick up where someone else left off? Our application isn't big enough that we are in trouble, I just think this is a good wakeup call.