[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>

docs/source/faqs.md

@@ -270,3 +270,23 @@ bash tools/install_flash_infer_attention_score_ops_a2.sh
 
 **NOTE**: Don't set `additional_config.pa_shape_list` when using this method; otherwise, it will lead to another attention operator.
 **Important**: Please make sure you're using the **official image** of `vllm-ascend`; otherwise, you **must change** the directory `/vllm-workspace` in `tools/install_flash_infer_attention_score_ops_a2.sh` or `tools/install_flash_infer_attention_score_ops_a3.sh` to your own, or create one. If you're not the root user, you need `sudo` **privileges** to run this script.
+
+### 23. How to set `SOC_VERSION` when building from source on a CPU-only machine?
+
+When building from source (e.g. `pip install -e .`), the build may try to infer the target chip via `npu-smi`. If `npu-smi` is not available (common in CPU-only build environments), you must set `SOC_VERSION` manually before installation.
+
+You can use the defaults from `Dockerfile*` as a reference. For example:
+
+```bash
+# Atlas A2
+export SOC_VERSION="ascend910b1"
+
+# Atlas A3
+export SOC_VERSION="ascend910_9391"
+
+# Atlas 300I
+export SOC_VERSION="ascend310p1"
+
+# Atlas A5 (Ascend 950 series)
+export SOC_VERSION="<value starting with ascend950>"
+```