Clarify inputs/outputs on IBM Circuit Function page

[jyu00]

URL to the relevant documentation

https://docs.quantum.ibm.com/guides/ibm-circuit-function

Select all that apply

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• out-of-date content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

Describe the fix.

• The Overview section talks about abstract PUBs as the input and links to PUBs description under primitives. But it doesn't say anything about what abstract PUBs mean (e.g. how it's different from what Qiskit Runtime Estimator takes).
• There is no explicitly section on output format. It's only mentioned under the example.

[abbycross]

Will check in with author of page!