Indicate nan handling in Ordinal docstring

[tamargrey]

In using the Ordinal.order property as part of implementing alteryx/evalml#1389, I ended up digging into Ordinal's implementation to remind myself how nans are handled. It could be useful to others in the future to have a sentence or two in the docstring explaining how nans in Ordinal columns get handled (they aren't treated as categories) and why (because you can't include a nan in setting the categories of the category dtype in pandas).