[ServiceBus] update docs

[swathipil]

similar to: #16094 --> PR for EH: #16215

• In the Event Hubs README, under section Create client using the azure-identity library, add the azure-identity installation step: pip install azure-identity
• we also need to mention async transport support (aiohttp) if people want to use azure-identity in async world, could just be a short sentence.