Although it's important to document a given action, I think it might be equally important to document why you didn't do a certain action.
Often when you're involved with a problem, you have a good understanding of all the pros and cons. You've gone down some paths, some things have worked, and other things haven't. Usually, you find a solution that fulfils all your needs at the point in time. So you run with it and document why you went with X.
However you should also document why you didn't do Y. In the future, someone will ask the question. If it's documented, you can just respond with the knowledge you had captured previously. If it's not documented, you often have to re-immerse yourself in the problem space again, action some work, and eventually hit the same roadblock you did months ago. Don't waste that time.