[SPARK-50138][PYTHON][DOCS][TESTS] Refine docstring for basic functions

### What changes were proposed in this pull request?
Refine the docstring for basic functions

### Why are the changes needed?
1, make example copy-pasteable;
2, clarify `str` is a column name or literal string;
3, `show` both the input columns and output columns, to make the projection more clear;
4, improve the test coverage by adding more doctests;

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

### How was this patch tested?
ci

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

Closes apache#48669 from zhengruifeng/py_doc_1.

Authored-by: Ruifeng Zheng <ruifengz@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>