unique docstring extend #13565 unique datetime tz issue
 closes  #13565
whatsnew
extended doc string for unique addressing the issue with timezone aware datetimes ( see #13565 )

not this not correct. documenting this is silly when it can easily be fixed.
 I thought it can't be fixed easily / its the expected behavior (your own words) so i added a the docstring. 
 well its the 'expected behavior' because of numpy silliness
 @paulgueltekin Thanks for the PR! I think it would be good to document this if we leave it as is (even when it is due to limitations of numpy). However, we are now thinking that we will return an object array of Timestamps (which can still hold the timezone information), and in that case this PR is not needed (see PR https://github.com/pydata/pandas/pull/13979#issuecomment-241200579)
