[TensorRT EP] Update instruction of `user_compute_stream` (#20343)
### Description
<!-- Describe your changes. -->
Update instruction and add sample code
### Preview the change:
https://yf711.github.io/onnxruntime/docs/execution-providers/TensorRT-ExecutionProvider.html#execution-provider-options
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->