add docs for KUNIT setting

overlay/default.nix

@@ -81,6 +81,23 @@ in
     };
     argsOverride = {
       structuredExtraConfig = with prev.lib.kernel; {
+        # The perl script to generate kernel options sets unspecified
+        # parameters to `m` if possible [1]. This results in the
+        # unspecified config option KUNIT [2] getting set to `m` which
+        # causes DRM_VC4_KUNIT_TEST [3] to get set to `y`.
+        #
+        # This vc4 unit test fails on boot due to a null pointer
+        # exception with the existing config. I'm not sure why, but in
+        # any case, the DRM_VC4_KUNIT_TEST config option itself states
+        # that it is only useful for kernel developers working on the
+        # vc4 driver. So, I feel no need to deviate from the standard
+        # rpi kernel and attempt to successfully enable this test and
+        # other unit tests because the nixos perl script has this
+        # sloppy "default to m" behavior. So, I set KUNIT to `n`.
+        #
+        # [1] https://github.com/NixOS/nixpkgs/blob/85bcb95aa83be667e562e781e9d186c57a07d757/pkgs/os-specific/linux/kernel/generate-config.pl#L1-L10
+        # [2] https://github.com/raspberrypi/linux/blob/1.20230405/lib/kunit/Kconfig#L5-L14
+        # [3] https://github.com/raspberrypi/linux/blob/bb63dc31e48948bc2649357758c7a152210109c4/drivers/gpu/drm/vc4/Kconfig#L38-L52
         KUNIT = no;
       };
     };