Add docs about new rule slugs

[frozzare]

Example:

repeater -> row -> group -> radio (value to check)
repeater -> row -> string (rule against radio property)

To check to you can't just write radio as rule slug, you have to write group.radio since the property is in a deeper than the string property.

Source:

• wp-papi/papi@f99db36
• wp-papi/papi@12f3024