docs: Add link to Safelisting specific utilities from upgrade guide

[CanRau]

Think it'd be helpful to have the @source inline feature mentioned in upgrade guide instead of it just saying safelist doesn't exist anymore (this might be a newer feature than when the docs were written).

Maybe could've been a little section or something, but wasn't sure how to do it best, and I think like this is better than not at all 🤓

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[rozsazoltan]

I believe people mostly use this feature in harmful ways, so I was glad it wasn't included in v4. That said, its not a bad thing that the option is available again from v4.1 - though it will likely lead to many projects where an unnecessary number of classes end up in the compiled CSS, instead of writing more dynamic classes tailored to their needs.

e.g.: tailwindlabs/tailwindcss#15291 (comment)

By the way, it's easy to find.

[thecrypticace]

I believe people mostly use this feature in harmful ways, …

With the regex version of safelist it was very easy to cause problems. You can definitely still generate way too much CSS than you need with the brace expansion syntax but it's a lot more "visible" than it was previously. IntelliSense will even tell you how many classes you're generating above each use of @source inline(…) and give a very rough estimate the amount of generated CSS if we think it'll get large enough:

I do think it's reasonable to link to the @source inline(…) docs from there though for the people who need it.

[thecrypticace]

I tweaked the wording a bit and removed the change that removed safelist from the list of unsupported config options. The safelist config option still isn't technically supported because the mechanism we use in v4 through @source inline(…) is different.