Business details:
* Product purpose - What it does.
* Business justification - Much money it generates or saves the company.
* Product Owner
* Business alignments/dependencies
Technical stuff:
* Describe what the product actually does, as opposed to what it claims to do.
* List all build and installation steps.
* Document how to run tests.
* Document technical requirements and business flow through the application.
* Try to autogenerate the areas of documentation that are most likely to go out of date. I wrote a build step in my Node.js app that does this so I don't have to manually maintain a good portion of the documentation.