docs: NVIDIA-INSTRUCTIONS.md: how to test the NVIDIA fix on the real PC, read the results and undo it

TODO-nvidia.md

@@ -1,6 +1,6 @@
 # TODO: NVIDIA fix for gaming mode (branch `feature/nvidia-gaming-fix`)
 
-Delete this file, `NVIDIA-RESEARCH.md` and `.no-release-yet` when the feature is released.
+Delete this file, `NVIDIA-INSTRUCTIONS.md`, `NVIDIA-RESEARCH.md` and `.no-release-yet` when the feature is released.
 Written 2026-10-01 so a new session can pick this up.
 
 ## Problem
@@ -70,7 +70,8 @@ needs a test on a real older card. Design for the toggle: on by default for any
 what it does, `tests/nvidia-hardware-test.sh` reports GPU and driver so results can be compared.
 
 ## Still to do
-1. [ ] **On the NVIDIA PC** (only place the real fix can be judged), from this checkout:
+1. [ ] **On the NVIDIA PC** (only place the real fix can be judged; the full steps, how to read the results and the undo
+   are in `NVIDIA-INSTRUCTIONS.md`), from this checkout:
    `bash tests/nvidia-hardware-test.sh check` (before), `... apply` (asks sudo), reboot,
    `... check` (after: `nvidia_drm modeset/fbdev = Y`), boot gaming mode, then
    `... visual` (was the picture clean?). Everything goes to `~/steamify-nvidia-report.txt`: