DOC: Series.map na_action confusion

A user on [stackoverflow](http://stackoverflow.com/questions/39461328/in-pandas-what-does-the-na-action-parameter-to-series-map-do/39461948?noredirect=1#comment66278752_39461948) was confused about the use of na_action in Series.map and so I came up with a simple example to explain it. @jorisvandenbossche asked me to submit a PR, so I created this issue to log the issue and will submit a PR now. 
