Document global.config

[lmiller1990]

We support config but we did not document it

It's here https://github.com/vuejs/vue-test-utils-next/blob/e5aee88ec6437a7d28dd0a4a9d0e84bbe3a8df40/src/types.ts#L24

[afontcu]

Do you believe this is enough? https://vue-test-utils.vuejs.org/v2/api/#global-config

[lmiller1990]

Yes!