Information about parent-child migration when removing native modifier

[sfreytag]

This PR implements the documentation suggestion contained within this GitHub issue comment:

https://github.com/vuejs/core/issues/4566#issuecomment-917997056

It explains how to migrate both the parent and child components for the removal of .native. In the Vue 3 migration build you also need to update the child to work with this change. This info is currently missing from the docs, but is well explained in the above GitHub comment. Therefore this PR does what is hopefully the obvious and helpful thing, and copies the comment into the docs.

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