MINOR: [Python] Add docs for missing type checking functions

[judahrand]

Rationale for this change

These appear to be missing and I only found them by accident!

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[AlenkaF]

Thanks @judahrand !

I am a bit confused with is_run_end_encoded entry. The CI doesn't complain and the diff from this PR doesn't show but when I look at the dev docs I can see the is_run_end_encoded already present after is_nested:
https://github.com/apache/arrow/blob/main/docs/source/python/api/datatypes.rst#type-checking

Could we therefore remove is_run_end_encoded and also add is_fixed_size_list() and is_duration() - I think they are also missing from the list.

[mapleFU]

LGTM!

[AlenkaF]

Thanks!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 6 benchmarking runs that have been run so far on merge-commit 5b9f4b9.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 6 possible false positives for unstable benchmarks that are known to sometimes produce them.