Explain how to use validators correctly

[dcastil]

In #418 (comment) @ozguruysal used the entire class in a validator which won't get the results he expected.

const twMerge = extendTailwindMerge({
  extend: {
    classGroups: {
      "font-size": [(classPart: string) => /^text-label-*/.test(classPart)],
    },
  },
})

I should write about how rules from the config are applied and in which order and derive from that how to best use validators.