DOC: Explain pivot vs. pivot_table (#6950)
 - [x] closes #6950



## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14650?src=pr) is 85.28% (diff: 100%)

> Merging [#14650](https://codecov.io/gh/pandas-dev/pandas/pull/14650?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14650   diff @@
==========================================
  Files           140        140          
  Lines         50693      50693          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43235      43234     -1   
- Misses         7458       7459     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [a7604fc...e98b27d](https://codecov.io/gh/pandas-dev/pandas/compare/a7604fcd8e105221c5cd5d469be9a3a308325631...e98b27d220a618d9c9f8536f081d4dc48181203f?src=pr)
 @jorisvandenbossche @jreback  Thanks for the feedback. Implemented the changes you guys suggested. 
 @mroeschke Thanks a lot! (feel free to do more PRs with doc improvements :-))
