BUG, DOC: Fix inconsistencies with scalar na_values in read_csv
1) Update documentation to state that scalars are accepted for `na_values`
2) Accept scalars for the values when a dictionary is passed in for `na_values`

Closes #12224.

pls update io/excel as well as I believe uses same code
 @jreback : Good point.  Fixed.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14056?src=pr) is 85.27% (diff: 100%)

> Merging [#14056](https://codecov.io/gh/pydata/pandas/pull/14056?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14056   diff @@
==========================================
  Files           139        139          
  Lines         50440      50442     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43012      43015     +3   
+ Misses         7428       7427     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [ae4ffac...7f253f2](https://codecov.io/gh/pydata/pandas/compare/ae4ffac560f9f6fbf1139dd4f391da52d12cdcce...7f253f235772107775ca9bd60df0c50cebe399d6?src=pr)
 Thanks!
