Migration from Vue 2 doesn’t mention .sync modifier

[palant]

I figured this out from a code comment but “Migration from Vue 2” document doesn’t seem to mention this change: the .sync event modifier is gone and replaced by v-model argument. So what was <component :prop.sync="value"> with Vue 2, should be <component v-model:prop="value"> with Vue 3.

For reference, this is mentioned in the code here: https://github.com/vuejs/vue-next/blob/052a621762c5f7c420464747ebbbed27c7350593/packages/compiler-core/src/transforms/vBind.ts#L22

[skirtles-code]

It's mentioned as part of https://v3.vuejs.org/guide/migration/v-model.html. I think we might want to improve the wording of the link from the introduction page so that it acknowledges the removed features. If you don't know that it's replaced by v-model then there's nothing to hint that the v-model page is the one you want.