[SPARK-50408][PYTHON][DOCS] Refine the docstring for datetime functio…

…ns - part 4

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

### Why are the changes needed?
to improve the docs and test 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 #48953 from zhengruifeng/py_doc_9.

Authored-by: Ruifeng Zheng <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>