[Doc] add faqs:install vllm-ascend will overwrite existing torch-npu (#3245)

### What this PR does / why we need it?
add faqs:install vllm-ascend will overwrite existing torch-npu

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.10.2
- vLLM main:
vllm-project/vllm@releases/v0.11.0

Signed-off-by: weiguihua2 <[email protected]>

Author: weiguihua2

docs/source/faqs.md

@@ -211,3 +211,6 @@ Recommended mitigation strategies:
 
 Root cause analysis:
 The current stream requirement calculation for size captures only accounts for measurable factors including: data parallel size, tensor parallel size, expert parallel configuration, piece graph count, multistream overlap shared expert settings, and HCCL communication mode (AIV/AICPU). However, numerous unquantifiable elements - such as operator characteristics and specific hardware features - consume additional streams outside of this calculation framework, resulting in stream resource exhaustion during size capture operations.
+
+### 21. Installing vllm-ascend will overwrite the existing torch-npu package？
+Installing vllm-ascend will overwrite the existing torch-npu package. If you need to install a specific version of torch-npu, you can manually install the specified version of torch-npu after installing vllm-ascend.