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

[jamesopti]

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

[Tamara-Barum]

Internal Ticket: [FSSDK-8657]