DOC: add examples to get_indexer_non_unique
- [X] closes #50132
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Updated docstring for `Index.get_indexer_non_unique`. Added examples to clarify `Index.get_indexer_non_unique` behavior.
> thanks for working on this

Thank you @MarcoGorelli for reviewing my PR. Some tests in CI failed, I am not sure if it’s related to my changes. Locally (on Linux) test `pandas/tests/indexes/object/test_indexing.py` passes. My local python version in mamba environment is `3.8.15`. It’s interesting that Python `3.11` is used in CI.