Document `view_duration_s` in api_json.log

What does this MR do and why?

view_duration_s in api_json.log is quite confusing. It looks like representing the duration of the payload serialization process in API, however, it turned out to be including multiple factors.

For avoiding the confusion, we should document this behavior.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.