clarified the documentation for DF.drop_duplicates
I hit an issue with a time series index where I wanted to keep duplicate data with different time values and only delete rows with the same time and columns. This documentation change would have saved me a lot of time.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @mjmdavis! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on February 01, 2019 at 17:43 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=h1) Report
> Merging [#25056](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/da5f5eb6c11140a2bb9f84049d30ba7d89bdbfd0?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25056/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25056      +/-   ##
==========================================
- Coverage   92.37%   92.36%   -0.01%     
==========================================
  Files         166      166              
  Lines       52403    52403              
==========================================
- Hits        48405    48404       -1     
- Misses       3998     3999       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.79% <ø> (ø)` | :arrow_up: |
| #single | `42.87% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25056/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.82% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25056/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=footer). Last update [da5f5eb...30d7ae1](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=h1) Report
> Merging [#25056](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/da5f5eb6c11140a2bb9f84049d30ba7d89bdbfd0?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25056/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25056   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52403    52403           
=======================================
  Hits        48405    48405           
  Misses       3998     3998
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.79% <ø> (ø)` | :arrow_up: |
| #single | `42.87% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25056/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.82% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=footer). Last update [da5f5eb...a1dbe83](https://codecov.io/gh/pandas-dev/pandas/pull/25056?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mjmdavis  Thanks @jreback!