DOC: Added additional example for groupby by indexer.
- [x] closes #13271 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Added more documentation and examples to clarify resampling as whole.
 @pfrcks Added a few more comments.
 @jorisvandenbossche kindly go through the changes and comment.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13276?src=pr) is **84.33%**

> Merging [#13276](https://codecov.io/gh/pydata/pandas/pull/13276?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.15%**

``` diff
@@             master     #13276   diff @@
==========================================
  Files           138        138          
  Lines         50581      51107   +526   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42582      43103   +521   
- Misses         7999       8004     +5   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [e0a2e3b...39b7fac](https://codecov.io/gh/pydata/pandas/compare/e0a2e3bc51f9e178f72c44e6de06700ee0bf31c6...39b7fac3cdb5ce6c1f81823012c759ee2acd01b9)
 @jorisvandenbossche Sorry, I overlooked that. Have made the necessary changes. Please look and comment.
 @pfrcks Thanks!
