DOC: resample warnings
- [ ] closes #13618, closes #13520
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Prevents warnings triggered by IPython by matching against `'_repr'` or `'_ipython'`, and expands the `resample` warning message to indicate what caused the deferred object to be materialized.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13675?src=pr) is **84.38%**

> Merging [#13675](https://codecov.io/gh/pydata/pandas/pull/13675?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **0.16%**

``` diff
@@             master     #13675   diff @@
==========================================
  Files           141        142     +1   
  Lines         51185      51225    +40   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43275      43226    -49   
- Misses         7910       7999    +89   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [506520b...c1f7727](https://codecov.io/gh/pydata/pandas/compare/506520bd35331aa82db50686c07d96594cac0c10...c1f77271ad77c87cd590a14fc120833e6a185db3?src=pr)
 lgtm. I would add at the very least a bug fix note. I don't think anything is warranted unless you think a user would be suprised by the change in deprecation warning.
 @jreback  - added a doc note - not sure why travis didn't run, but that's the only change.  I didn't think there needs to be any doc change on the deprecation warning, it's basically saying the same thing, just more verbose.
 thanks!
