short answer:

Don’t “fill in” everything. Only what your stakeholders really need.

long answer:

Everything you document will cause you maintenance work later. With each piece of documentation, you are potentially creating more work in the future. Alistiar Cockburn remarked: “Our main goal in product development is to produce deliverable products. But our secondary goal is to be properly positioned for the next releases.”

So please only document things, facts and reasons that you think are valuable enough or that anyone would absolutely want documented. Make sure that “someone” can also provide you with the resources for the documentation effort (see also [AJFZ22] Chapter 75: Waisenkinder (Orphaned Deliverables)).