Add a warning about including `mintAdminToken`

[reimertz]

Thanks for these amazing snippets but I feel like there should be a warning about including this in any production environment since the combination of mintAdminToken and recursiveDelete would enable to anyone blasting an entire production database.

I suggest you create an example that isn't copy-pasteable.

snippets-node/firestore/solution-deletes/functions/index.js

Line 19 in ea3747e

exports.mintAdminToken = functions.https.onCall(async (data, context) => {