DOC: Correct uniqueness of index for Series
closes #7808

Just wanted to fix the docstring to reflect the fact that the index labels neither need to be unique ~~nor hashable~~.

It is true that labels no longer need to be unique, but they still must be hashable?
 No they don't need to be hashable currently:

```
In [5]: s = pd.Series ([1,2,3], index = ([1],[1],[1]))

In [6]: s
Out[6]:
[1]    1
[1]    2
[1]    3
dtype: int64
```

Do you think we should add a check for hashable? 
 try to index that and see what happens 
 @jreback lol i don't know, maybe some people use series without ever indexing them? otherwise do you think we should add a check for hashable values? Or maybe edit the doco to say "for the object to support indexing, the labels must be of a hashable type"?
 ```
In [4]: s = pd.Series ([1,2,3], index = ([1],[2],[3]))

In [5]: s.loc[[1]]
TypeError: unhashable type: 'list'

In [7]: s.index.inferred_type
Out[7]: 'mixed'
```

So we do actually check hashable, but its fairly lazy as its somewhat to expensive to do this. The point is that this is non-idiomatic, and though may work in parts of pandas is not supported, nor recommended in any way.

So if you'd like to update with a stronger warning would be great.
 @themrmax Can you update this? (leave out the hashable change for now, but you can keep the change regarding uniqueness) 
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14344?src=pr) is 85.21% (diff: 100%)
> Merging [#14344](https://codecov.io/gh/pandas-dev/pandas/pull/14344?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **0.04%**

```diff
@@             master     #14344   diff @@
==========================================
  Files           140        143     +3   
  Lines         50632      50800   +168   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43170      43289   +119   
- Misses         7462       7511    +49   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [96b364a...6462ef5](https://codecov.io/gh/pandas-dev/pandas/compare/96b364a4a337b608c92dd6d5a00ceedd80c29315...6462ef5277cd99efda3898a5e1482da3e23d11dc?src=pr) @themrmax Thanks! 
(I added one sentence from another PR for the same issue ("Non-unique index values are allowed") before merging) cool thanks @jorisvandenbossche yes that's clearer now !