Improve discussion and examples for rule to avoid unowned captures

[calda]

Summary

This PR improves the discussion and examples for the "avoid unowned self" rule:

• The examples now use unowned self and weak self, which feels less contrived than the previous examples
• We now include discussion around explicitly capturing the instance members used in the closure (e.g. [planet] instead of [weak self] / self?.planet).

---

unowned captures are unsafe because they will cause the application to crash if the referenced object has been deallocated.

// WRONG: Crashes if `self` has been deallocated when closures are called.
final class SpaceshipNavigationService {
  let spaceship: Spaceship
  let planet: Planet
  
  func colonizePlanet() {
    spaceship.travel(to: planet, onArrival: { [unowned self] in
      planet.colonize()
    })
  }
  
  func exploreSystem() {
    spaceship.travel(to: planet, nextDestination: { [unowned self] in
      planet.moons?.first
    })
  }
}

weak captures are safer because they require the author to explicitly handle the case where the referenced object no longer exists.

// RIGHT: Uses a `weak self` capture and explicitly handles the case where `self` has been deallocated
final class SpaceshipNavigationService {
  let spaceship: Spaceship
  let planet: Planet
  
  func colonizePlanet() {
    spaceship.travel(to: planet, onArrival: { [weak self] in
      guard let self else { return }
      planet.colonize()
    })
  }
  
  func exploreSystem() {
    spaceship.travel(to: planet, nextDestination: { [weak self] in
      guard let self else { return nil }
      return planet.moons?.first
    })
  }
}

Alternatively, consider directly capturing the variables that are used in the closure. This lets you avoid having to handle the case where self is nil, since you don't even need to reference self:

// RIGHT: Explicitly captures `planet` instead of capturing `self`
final class SpaceshipNavigationService {
  let spaceship: Spaceship
  let planet: Planet
  
  func colonizePlanet() {
    spaceship.travel(to: planet, onArrival: { [planet] in
      planet.colonize()
    })
  }
  
  func exploreSystem() {
    spaceship.travel(to: planet, nextDestination: { [planet] in
      planet.moons?.first
    })
  }
}

[jqsilver]

LGTM!