docs: stop saying the same thing twice

The hardware section repeated what 'What you need' already said and what
troubleshooting now covers. It keeps only what neither says: that the multi-GPU
paths assume eight XGMI-connected devices, and that the single place ROCm
appears at all is the optional kernel rebuild.

Author: maincode-dave

README.md

@@ -563,29 +563,22 @@ what that costs a real model isn't measured here.
 
 ---
 
-## Hardware and requirements
+## Hardware
 
-The CPU lane runs on any x86-64 Linux host with Docker or Podman, and needs no
-GPU at all.
+[What you need](#what-you-need) covers the short version. Two things it leaves
+out.
 
-The GPU lane needs an AMD GPU reachable at `/dev/kfd` and `/dev/dri`. The kernels
-are compiled for gfx950, which is MI355X on CDNA4, and other architectures will
-load the wrong ISA. Multi-GPU paths assume 8 XGMI-connected devices.
+The multi-GPU paths, meaning the collectives and anything measured against RCCL,
+assume eight XGMI-connected devices. Everything else runs on one.
 
-No ROCm is needed anywhere, at build time or run time, and the devcontainer image
-contains none of it.
+No ROCm is needed anywhere, at build time or at run time, and the devcontainer
+image contains none of it. The one place ROCm appears at all is
+`kernels/build.sh`, which borrows an amdgpu-capable clang to recompile the kernel
+source, and you only need that if you change a kernel. The compiled object is
+committed.
 
-The host user has to be in the `render` group before the container starts so that
-`keep-groups` can hand through access to the device nodes. If `just probe`
-reports the device unavailable, reconnect after the group change takes effect.
-
-One thing that catches people out. The `--node <n>` argument throughout is a KFD
-topology node id rather than an ordinal GPU index, and `mainarch probe` lists the
-ones on your machine. The examples all use `--node 2` because that's where the
-first GPU lands on the reference host, and yours may well differ. `just demo`
-works it out for you.
-
-Fuller detail and the validation tiers live in `docs/hardware-support.md`.
+`docs/hardware-support.md` has the validation tiers and what each one actually
+proves.
 
 ---