feat(custom extension ref): document defaultNumberOfRetries property

-- adds documentation for asyncBefore, asyncAfter, exclusive of
the following elements:
- Intermediate Throw Events
- Intermediate Catch Events
- Start Events
- EndEvents
- Manual Task
- Exclusive Gateways
- Inclusive Gateways
- Event-Based Gateways (asyncAfter is not supported at the moment)

Related to: #CAM-2723