[SPARK-50390][PYTHON][DOCS] Refine the docstring for datetime functio…

…ns - part 2

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

### Why are the changes needed?
to improve docs and test coverage

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

### How was this patch tested?
updated doctests

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

Closes #48929 from zhengruifeng/py_doc_7.

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