### What this PR does / why we need it?
- Clarify that `SOC_VERSION` must be set when building from source in a
CPU-only environment where `npu-smi` is unavailable.
- Add concrete `SOC_VERSION` examples (A2/A3/300I/A5) and point users to
`Dockerfile*` defaults.
- Improve the `setup.py` error message so users get actionable guidance
when `SOC_VERSION` is missing.
Fixes#6816.
### Does this PR introduce _any_ user-facing change?
- Yes. Documentation is updated and the build-time error message is more
informative.
### How was this patch tested?
- (Local) Syntax check: `python -m compileall setup.py`.
- vLLM version: v0.17.0
- vLLM main:
4034c3d32e
Signed-off-by: NJX-njx <3771829673@qq.com>
bb506a1c99