docs(ssr): properly handle vite.middlewares after restart

[patak-dev]

Description

See:

• fix: use latest module graph in transform middleware #14892
• fix: server.headers after restart in middleware mode #14905

vite.middlewares is updated after a restart in middlewareMode: true. We need to recommend an indirection so users always get the latest middlewares, as proposed by @bluwy here

We could try to make vite.middlewares more magical and avoid reassigning in the future, but it isn't bad either that users know the middlewares change after a restart explicitly.

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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, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.