DOC: Added docstrings to fixtures defined in XML I/O, util and pandas modules
Partially addresses: #19159

Co-authored-by: @GeorgeJuniorGG and @MylenaRoberta.
> Thanks for the contribution @CristianoS16, good work, I added some suggestions that you can have a look at. Note that I only add the suggestions once, but for example having the constant values `True` or `False` with backticks is something that can apply to several docstring in your changes.

Thanks @datapythonista for the suggestions. I think we fixed it! Thank you @CristianoS16 for the contribution, nice work, and sorry for the delay.