ci: document arm oddity in build-rules

There's an oddity in the .lava-test:amd64 build rules; we depend on and
use the ARM images instead of the AMD64 images. The reason for this is
kind of silly; we need the Docker image to match the architecture of the
runner, which happens to be ARM.

So this isn't at all about the target architecture, but more of a Docker
detail.

Hopefully documenting this will prevent others from spending time being
puzzled about this in the future.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8967>

.gitlab-ci/lava-gitlab-ci.yml

@@ -133,11 +133,11 @@ kernel+rootfs_armhf:
     TEST_SUITE: "deqp"
   extends:
     - .use-x86_build-base # for same $MESA_BASE_TAG as in kernel+rootfs_amd64
-    - .use-arm_build
+    - .use-arm_build # ARM because it must match the architecture of the runner
     - .lava-test
   needs:
     - kernel+rootfs_amd64
-    - arm_build
+    - arm_build # ARM because it must match the architecture of the runner
     - meson-testing
 
 .lava-traces-base: