Comment of featureFlagOptions doesn't match the real behavior

[zhiyuanliang-ms]

https://github.com/Azure/AppConfiguration-JavaScriptProvider/blob/main/src/featureManagement/FeatureFlagOptions.ts#L22

https://github.com/Azure/AppConfiguration-JavaScriptProvider/blob/main/src/AzureAppConfigurationImpl.ts#L626

Comment: #65 (comment)

[zhiyuanliang-ms]

JS provider will throw exception if no selectors is specified for feature flags instead of load all feature flags with null label.

This is inconsistent with all of our other providers.

Discussed offline with @zhenlan . We should make it consistent.