DOC: Correct uniqueness of index for Series (#7808)
 - [X] closes #7808



@mroeschke Thanks for the PR. However, there was already another PR for the same change, see https://github.com/pandas-dev/pandas/pull/14344. So I merged that one.  
That PR was apparently not correctly linked to the issue #7808 (added that now), so it was not clear that there was already a PR. Sorry for that! But I included the "Non-unique index values are allowed." that you included in this PR, in the other one. No worries, thanks for the heads-up!