[Doc] Update installation instructions

[toolsmanhehe]

What this PR does / why we need it?

When we install vllm-ascend in a non-pure environment, the project may not run as expected due to interference from older versions.

Does this PR introduce any user-facing change?

It merely adds relevant installation tips.

How was this patch tested?

I tested the installation steps in a non-pure environment on GitCode and found this issue.

• vLLM version: v0.11.0rc3
• vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0

[gemini-code-assist]

Code Review

This pull request updates the installation documentation to help users avoid issues with previous installations and to clarify expected runtime messages. The changes are helpful, but there are a few areas where the wording can be improved for clarity and correctness. I've suggested changes to correct an erroneous package name and to rephrase some comments in a code block to be more direct and grammatically correct. These improvements should help prevent user confusion during the setup process.

[github-actions]

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