Document object field options

[jossmac]

Add a section to the object field documentation detailing the options argument:

type ObjectFieldOptions = {
  label: string;
  description?: string;
}

Where usage looks like:

fields.object(
  {
    a: fields.text({ label: 'A' }),
    b: fields.text({ label: 'B' }),
  },
  {
    label: 'Group label',
    description: 'Group description',
  }
)

Relates to #641