[SPARK-50390][PYTHON][DOCS] Refine the docstring for datetime functions - 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 <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: zhengruifeng