Properly format warning blocks in the numpy/xarray wrappers

Made this issue to move along with #40 for now, but keep this in mind. 

@TomNicholas commented in https://github.com/xgcm/aerobulk-python/pull/15#discussion_r897291999, that our warnings should be formatted according to RST. But I am confused why we are using RST in the docstrings. Is there some example that we could use as a reference?

