update io documentation
The HDFS dropna=True parameter does not drop the NaN values. Perhaps this is a bug to the core Pandas, or really a typo in the documentation

- [X] closes #35719
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The way you're changing the documentation, setting `dropna=True` keeps NA values. That isn't correct, I believe this is a bug, not an issue with the docs. Ah, glad this bug is found soon, 'cause I myself kinda doubt that the errata comes from the docs, but apparently it's not the case. Thanks to @XiaozhanYang observations. @simonjayhawkins Thanks for the quick updates with @rhshadrach. I am actually new to contributing to open source. I myself am quite familiar with Python. Perhaps with regards to this bug, you have any suggestion or tips that would allow helping contribute? thanks in advanced, really love pandas hahaha. @grrlic in the first instance, if you are unfamiliar with the codebase, I would probably bisect the regression, see #35685 to get a idea of where a fix should be applied.

although for this case, it might be quite straightforward to follow the code from `to_hdf` (I notice from git blame that there we changes to the signature in #29957) It appears that dropna is not passed onto `store.put` in the if append else block