Document usage of new formfield props as an example

[Skaiir]

Is your feature request related to a problem? Please describe.

We've made some changes to how form-fields work by extracting a few properties to be computed in the form-field parent class. This may be worth documenting here as per bpmn-io/form-js#808 (comment)

Describe the solution you'd like

• will add details later :)