Clarify usage

[beckykd]

URL to the relevant documentation

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

Select all that apply

• [ ] typo
• [ ] code bug
• [ ] out-of-date content
• [ ] broken link
• [x] other

Describe the fix.

Customers are still unsure of how usage is calculated; especially for sessions. For example, what about when your session is paused due to the ITTL being reached? Add further clarifications if possible.