docs: clarify code comments in optimize-missing-deps

[benmccann]

Description

I didn't understand that "missing" meant not discovered. Also, the sentence "this will import missing deps nest built-in deps" seems like it has a typo because it doesn't make sense

Please check that what I've written is correct

Additional context

This test was failing when I was working on another change and I wanted to better understand it

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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.

[ygj6]

I'm sorry I wrote this confusing note, which contains a stale story about "built-in deps". Your changes are correct to make it clearer, thank you!

In addition, the main purpose of optimize-missing-deps is to test that _registerMissingImport is not triggered during SSR. You can see the reason and my detailed description at #5017 (comment). Hope it helps you understand.

[brillout]

@ygj6 Do you know why the test is failing (https://github.com/vitejs/vite/pull/6698/checks) when externalizing SSR dependencies (#6698)?

[bluwy]

Wonder if this description is more concise. I find the current one a bit off 😅