docs: update WSL2 watch limitation explanation

[sapphi-red]

Description

This PR adds the following warning for WSL2 when usePolling is not set. I'm not sure if it is good to have this warning for all WSL2 users.

(!) Default file system watching might not work with your setup due to the limitation of WSL2. HMR and other features will not work when file system watching is not working. To suppress this warning, set true or false to "server.watch.usePolling". More information: https://vitejs.dev/config/server-options.html#server-watch

Also this PR improves the document about that.

This also applies to docker with WSL2 backend and docker running on WSL2 directly.

refs #4116

Additional context

At first I thought it is able to detect the case if file system watching works or not.
But after implemented that, I noticed that the document is not correct.
The current explanation is:

When running Vite on Windows Subsystem for Linux (WSL) 2, if the project folder resides in a Windows filesystem

But actually it is:

When running Vite on WSL2 and a file is edited by Windows applications (non WSL2 process)

The former one is possible to detect, but the latter one is not possible since Vite runs as a WSL2 process or Windows process.
We could start a powershell process from Vite but that won't work with Docker(WSL2 backend).

One way to implement is to show this warning if any file change was not detected after some time.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• 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).
• Ideally, include relevant tests that fail without this PR but pass with it.

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[bluwy]

Neat!

[sapphi-red]

I reconsidered that the warning is too noisy and removed it.

[patak-dev]

Maybe there could be an short entry in troubleshouting pointing to these sections too?

Ah, I see there is already a section https://vitejs.dev/guide/troubleshooting.html#vite-does-not-detect-a-file-change