[API-42062] Update /power-of-attorney-requests/decide docs

[tycol7]

Summary

Add participantId and representativeId to the /decide request body in v2 staging docs. See #19362 for more.

Related issue(s)

API-42062

Testing done

Visual inspection.

Screenshots

What areas of the site does it impact?

v2 staging docs.

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

Requested Feedback

N/A

[rockwellwindsor-va]

Docs looked good to me