DOC: added example to Series.map showing use of na_action parameter (GH14231)
- [x] closes #14231
- [x] tests passed
- [x] passes `git diff upstream/master | flake8 --diff`

Added example to Series.map showing use of na_action parameter.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14232?src=pr) is 85.24% (diff: 100%)

> Merging [#14232](https://codecov.io/gh/pydata/pandas/pull/14232?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14232   diff @@
==========================================
  Files           140        140          
  Lines         50559      50559          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43099      43100     +1   
+ Misses         7460       7459     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [5e2f9da...b847717](https://codecov.io/gh/pydata/pandas/compare/5e2f9da6e8e713bd89cfe8760e63583ea7d29879...b847717103cb1d2e5ceddaae9abce2c7222e106f?src=pr)
 @shawnheide Thanks a lot!
