docs: add cautionary note for undocumented parameter usage

[rajghosh06-dev]

This PR updates the README.md under the section “Undocumented request params” to include a cautionary note about using undocumented parameters like baz. While the current example uses @ts-expect-error, it lacks guidance on the risks associated with undocumented API fields.

The new note:

• Encourages wrapping undocumented fields with @ts-expect-error and a clear comment.
• Warns that such fields are not part of the public API and may change without notice.
• Improves onboarding clarity and reviewer experience for contributors exploring advanced usage.

This change is purely documentation-related and does not affect runtime behavior or typings.