DOC: Clarify return type cases in pandas.unique
- [ ] closes #47426
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Hello @mitlabence! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/core/algorithms.py`](https://github.com/pandas-dev/pandas/blob/a13d55215d9bc8c7e971356c1946aa25ae032f18/pandas/core/algorithms.py):

> [Line 336:61](https://github.com/pandas-dev/pandas/blob/a13d55215d9bc8c7e971356c1946aa25ae032f18/pandas/core/algorithms.py#L336): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace

