docs: Update flex.md warn important (#15636)
docs: Update flex.md warn important (#15636)

## Description
Resolves laking documentation of the risk of using d-flex;
resolves https://github.com/vuetifyjs/vuetify/issues/14773

## Motivation and Context
So far there's no info in Vuetify documentation on the risk d-flex
affecting behaviours of v-show and others.
fixes https://github.com/vuetifyjs/vuetify/issues/14773.
```vue
// Paste your FULL Playground.vue here
```
</details>

## Types of changes
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the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [x] Improvement/refactoring (non-breaking change that doesn't add any
features but makes things better)

## Checklist:
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
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- [x] The PR title is no longer than 64 characters.
- [x] The PR is submitted to the correct branch (`master` for bug fixes
and documentation updates, `dev` for new features and backwards
compatible changes and `next` for non-backwards compatible changes).
- [x] My code follows the code style of this project.
- [ ] I've added relevant changes to the documentation (applies to new
features and breaking changes in core library)
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