The best time to document an edge case is the time it is being written.
Writing test cases at the end of a project is too late, as you will likely have already forgotten the more pertinent cases. This applies to both automated and manual testing.
Adding to a QA document at the time of development helps non-technical team members to understand where customisations have been added and specific areas that need special care.
Furthermore, a list of these scenarios is great for penetration testing, providing invaluable information on where vulnerabilities may lie.