[DOC] Clarify the behavior of SetProviderAsync

[beeme1mr]

Change in the documentation

The SetProviderAsync behaves differently than nearly every other method in OpenFeature because it can throw if the underlying provider throws during its initiation. This was a conscious design choice documented in the spec (requirement 1.4.10). However, it's not obvious in our examples that an error can be returned while setting a provider.

Requirements

• Ensure that the XML Doc comments for the SetProviderAsync method clearly defines the behavior.
• Update the examples in the readme to use a try/catch when using SetProviderAsync.

[kylejuliandev]

I've created a PR for this (#449), let me know if this is what you had in mind 😊