docs: clarify JsonRpcProvider vs WebSocketProvider behavior

[stalim17]

This PR adds a short note to the documentation clarifying the behavioral difference between JsonRpcProvider and WebSocketProvider.

JsonRpcProvider relies on polling under the hood, while WebSocketProvider enables push-based subscriptions suitable for low-latency event streams such as pending transactions or log subscriptions.

This helps integrators choose the appropriate provider type when building real-time UX. Documentation-only change, no code modifications.