As some of you mentioned to retro spectically find as many document as possible and move forward from them but here is something which I would suggest as a plan to go for especially for legacy products to get approval.
So here is the flow. Correct me if wrong.

1. Get all documents available.
2. Perform Gap analysis.
3. Identify what data needed for fixing gaps.
4. Collect information as much as available to fill the gaps
5. Create design doc as per “Current” procedures and templates.
6. Perform reviews and approvals.
7. Note changes happened in the design
8. Documents changes as per procedures and version control
9. Perform reviews and approvals.

This plan covers most part of the documentation but things to looks out would be make a checklist of current document needed. Sometimes you wont needed a review and approval for some document like DMR, as the product is already being manufactured for a long time. so make a list of such document which doesn’t require approval so that you don’t waste time.