Document that the `custom()` function can return an object

[scharf]

for example:

custom() {
      if (this.value !== this.field('password').value) {
        return { type: "passwordMismatch", message: "Passwords do not match!" };
      }
    },

[sophos-rickc]

@scharf, this is a very nice callout! I was not aware that this is possible. I may use it.

[scharf]

It is now documented Validate one key against another.

[scharf]

ah -- I realized that it is not documented that it can return an object -- to verify this, it would also need a test....