Add: Callback Flush Timing - About the flush: sync

[niceplugin]

Add content that watchers.md : Callback Flush Timing - About the flush: sync option

Among the options used for watch, flush: 'sync' was not explained in the guide or API documentation, so I added it.

It need to check because my English is not good.

Hope this helps.

[netlify]

✅ Deploy Preview for vuejs ready!

Name	Link
🔨 Latest commit	e8ec147
🔍 Latest deploy log	https://app.netlify.com/sites/vuejs/deploys/62b6915793baba000849674e
😎 Deploy Preview	https://deploy-preview-1798--vuejs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[niceplugin]

It looks fine, but I only got the review label.
Is there anything more I should do?

[skirtles-code]

Thanks for the PR and sorry for the delay in getting back to you.

flush: 'sync' poses a documentation challenge. It's an 'escape hatch' feature, intended to be used in specific edge cases by advanced users. Such features are typically not covered in the main guides, or at least not the guides aimed at beginners. Usually such features are only documented in the API reference, or even omitted from the docs entirely in extreme cases. In this specific case we have the added complication that flush: 'pre' and flush: 'post' are mainstream, commonly used features that very much need to be covered in the basic guides.

I did raise this internally and the feeling seemed to be that we want to keep 'sync' out of the main guide. It's already mentioned in the API reference, but as you correctly pointed out there's no explanation about what it does.

Adding an explanation in the API reference is also a bit of a pain because the flush setting is relevant to various entries.

I've tried to strike a balance in #1895 (now merged), providing a brief introduction to flush: 'sync' and also trying to warn people away from using it. The explanation is under watchEffect, with links added from other places that have a flush setting.