docs: note api run in same process

[bluwy]

Description

Close #8204

Clarify when using createServer and build simultaneously on the same nodejs process

Additional context

Not sure of the perfect spot to put the note.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
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• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[bluwy]

I also noticed that using process.env.NODE_ENV in the docs breaks dev mode, but prod is not affected. Related issue: vuejs/vitepress#419

.