enhancement (OptimizelyExperiment): Clarify expected usage of default and log violations

## Summary

The `<OptimizelyExperiment>` component makes some assumptions about the `children` prop that we should document and warn about:

- Exactly 0 or 1 `<OptimizelyVariation default >` child components should be passed
- If 1 is passed, ensure it is the last child in the list
  - Upon finding one, the loop short circuits because it has found a match