DOC/TST: is pd.unique and the order it returns API?
Looking at the API docs, neither the function `pd.unique` nor the order of the unique values from `unique` is mentioned. I would like to:
1. Add `pd.unique` to API > General functions > Data manipulations
2. Note that `Series.unique()` and `unique()` return values in order of appearance
3. Add unit tests to verify the "order of appearance" nature `unique` (untested directly AFAICT, though I'm sure it's relied on implicitly)

Any objections? If not, I'll put together a PR.

This lack of documentation has caused some hesitation for relying on this functionality in Seaborn: https://github.com/mwaskom/seaborn/issues/361
