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:

Add pd.unique to API > General functions > Data manipulations
Note that Series.unique() and unique() return values in order of appearance
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: mwaskom/seaborn#361

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