Add disclaimer about circuit time

[jyu00]

URL to the relevant guide or resource

https://quantum.cloud.ibm.com/docs/en/guides/stretch
https://quantum.cloud.ibm.com/docs/en/guides/estimate-job-run-time

Select all that apply

• new content request
• typo
• code bug
• out-of-date content
• broken link
• other

Describe the fix or the content request.

Related to #3848.

The new feature that returns circuit timing information also returns the total circuit time. We should add disclaimers that this is NOT the time used for billing.

The circuit time is the total duration of the circuit once it's scheduled. We define "quantum time", however, as "time spent by the QPU complex to process your job", which includes time to load the payload, rep_delay between each shot loop, etc.

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