[Doc][Installation] Clarify SOC_VERSION for CPU-only source builds (#7278)

### 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>

setup.py

@@ -120,7 +120,12 @@ if not envs.SOC_VERSION:
         raise RuntimeError(
             "Could not determine chip type automatically via 'npu-smi'. "
             "This can happen in a CPU-only environment. "
-            "Please set the 'SOC_VERSION' environment variable to specify the target chip."
+            "Please set the 'SOC_VERSION' environment variable to specify the target chip, for example:\n"
+            '  - Atlas A2: export SOC_VERSION="ascend910b1"\n'
+            '  - Atlas A3: export SOC_VERSION="ascend910_9391"\n'
+            '  - Atlas 300I: export SOC_VERSION="ascend310p1"\n'
+            '  - Atlas A5: export SOC_VERSION="<value starting with ascend950>"\n'
+            "You can also refer to the SOC_VERSION defaults in Dockerfile*."
         )
     envs.SOC_VERSION = soc_version
 else: