Allow Record<string, string> as return type for DescriptionFunction

[franckmartin]

Currently, the only allowed return type for a DescriptionFunction is string.
This is a problem if you want to do something like this:

admin: {
  description: ({value}) => ({
    en: `${value.length} characters`,
    fr: `${value.length} caractères`,
  }),
},

The return type Record<string, string> should be allowed for such a function.