[MINOR][DOCS] Fix the type hints of `functions.first(..., ignorenulls)` and `functions.last(..., ignorenulls)`

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to fix the type hints of functions.first(..., ignorenulls) and functions.last(..., ignorenulls) to be properly bools.

Why are the changes needed?

To guide users about the correct types

Does this PR introduce any user-facing change?

Yes, it changes the user-facing documentation.

How was this patch tested?

CI in this PR should verify them.

Was this patch authored or co-authored using generative AI tooling?

No.