DOC: Clarified and expanded describe documentation
 - [x] passes ``git diff upstream/master | flake8 --diff``

As discussed in #14483.
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14995?src=pr) is 84.77% (diff: 100%)
> Merging [#14995](https://codecov.io/gh/pandas-dev/pandas/pull/14995?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **0.01%**

```diff
@@             master     #14995   diff @@
==========================================
  Files           145        145          
  Lines         51090      51129    +39   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43316      43343    +27   
- Misses         7774       7786    +12   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [eecfa88...8880a89](https://codecov.io/gh/pandas-dev/pandas/compare/eecfa8843a0c90b5424e9e0e4d353bbb3b78a671...8880a89889e16ced7c0b6201f40981b7e8b51b68?src=pr) I've just pushed an update that tried to address your requests, @jorisvandenbossche. Please let me know if there's more I can do. @jorisvandenbossche, I have just pushed a number of new changes to try to conform with your second round of requests. Please let me know if there is more I can do. Thank you for your thorough review.  @palewire Looks good! You only have a few PEP8 issues (which causes travis to fail):

```
pandas/core/generic.py:5222:80: E501 line too long (85 > 79 characters)
pandas/core/generic.py:5239:80: E501 line too long (81 > 79 characters)
pandas/core/generic.py:5320:80: E501 line too long (80 > 79 characters)
pandas/core/generic.py:5353:80: E501 line too long (81 > 79 characters)
```

If those are fixed this can be merged! I am ok with this as the docs for series are so close to dataframe its fine to combine.
 @jorisvandenbossche I have just now made and pushed the PEP8 corrections you requested.  @palewire Thanks!