docs: update When section of getStaticProps page

[sumiren]

getStaticProps will be called in conjection with fallback.

It's written in following page, but not in concerned page.
https://nextjs.org/docs/basic-features/data-fetching/get-static-paths

On my environment, pnpm lint detect some errors in files in pacakges and test.
I'm not sure whether CI ignores these errors, but at least it seems my change contains no error, so I checked checklist.

Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running pnpm lint
• The examples guidelines are followed from our contributing doc

[ijjk]

Thanks!