Fixed comment for pandas.unique
Timed pd.unique vs np.unique with 1_000 and 100_000 sequences lengths. In first case np.unique was faster, whereas in the second case pd.unique won.
Hence, I propose to make the doc a bit more accurate.

![image](https://user-images.githubusercontent.com/1339818/106612351-519ac100-6569-11eb-9388-1acfc39ef3a1.png)

thanks @Remit  @jreback This was merged directly to 1.2.x and milestoned 1.3

revert on 1.2.x and open PR against master?
 oh prob an accident @Remit if you'd like to re-submit this against master. #39643 closed by https://github.com/pandas-dev/pandas/pull/39643 > @Remit if you'd like to re-submit this against master.

Ok, did so. Thanks!