[SPARK-50159][PYTHON][DOCS][TESTS] Refine docstrings for aggregation …

…functions - part 1

### What changes were proposed in this pull request?
Refine docstrings for aggregation functions - part 1

### Why are the changes needed?
1, improve the documentation;
2, improve the doctest coverage;

### Does this PR introduce _any_ user-facing change?
doc-only changes

### How was this patch tested?
new doctests

### Was this patch authored or co-authored using generative AI tooling?
no

Closes #48694 from zhengruifeng/py_doc_3.

Authored-by: Ruifeng Zheng <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>