improve the warning in `inject`

I believe it's coming from this line:

https://github.com/vuejs/vue/blob/9e88707940088cb1f4cd7dd210c9168a50dc347c/src/v3/apiInject.ts#L69

I started seeing a lot of this warning after a recent upgrade. I thought it was my code but turned out to be Nuxt UI. Including the key in the warning would be helpful.