Update sessions usage info

[kt474]

URL, if applicable

https://docs.quantum.ibm.com/guides/execution-modes#sessions-versus-batch-usage

https://github.com/Qiskit/documentation/blob/main/docs/migration-guides/sessions.mdx

Describe the new content you are requesting.

In qiskit-ibm-runtime 0.30.0, we added a new Session.usage() and Batch.usage() method that can be used instead of session.details()["usage_time"]

If this new content request is accepted, do you want to write the content?

I will write a draft of the proposed content

[abbycross]

Thanks @kt474 !