BUG: fix Panel.fillna() ignoring axis parameter

This fixes #8251. It may need some more comprehensive tests but before I go too much farther I want to make sure the axis numbering scheme makes sense . The default behavior of `fillna` is to fill along `axis=0`. In the case of a DataFrame this fills along the columns. However, in the case of a Panel, filling along `axis=0` means you're filling along items. I'm not sure if that's the behavior that most users would think is the default. You might note that I also removed the references to what the behavior of each axis value is in the docstring since it gets a bit arbitrary when you consider both DataFrames and Panels.
It should still be functional for panels with mixed_type data. The way panel filling works is the panels are broken down into their constituent dataframes and then those are filled quasi-recursively. When the mixed-type dataframes come around the check will be performed and they'll be handled appropriately.

I will definitely add some times to make sure this is the case.