[Issue: #16416] Adding examples for some DataFrame methods
Adding examples for [`fillna`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.fillna.html), [`drop`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.drop.html), and [`sort_values`](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.sort_values.html).
 - [x] related to #16416 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

This looks great! Could you also add an example with a `method`? Maybe just `method='ffill'`. I'm also having some issues because when I build locally, the line, "Fill NA/NaN values using the specified method" seems to appear in the bottom of the documentation. I'm not sure why it doesn't stay at the top?

This line seems to be coming from here: https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L3433 @geoninja # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=h1) Report
> Merging [#16437](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/49ec31bbaeca81a6f58fc1be26fe80f3ac188cdd?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/16437/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16437   +/-   ##
=======================================
  Coverage   90.42%   90.42%           
=======================================
  Files         161      161           
  Lines       51023    51023           
=======================================
  Hits        46138    46138           
  Misses       4885     4885
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.26% <ø> (ø)` | :arrow_up: |
| #single | `40.17% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.69% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.13% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16437?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/16437?src=pr&el=footer). Last update [49ec31b...cc734ba](https://codecov.io/gh/pandas-dev/pandas/pull/16437?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/16437?src=pr&el=h1) Report
> Merging [#16437](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/97ad3fb9c87226ad983267e2891dbbf68432b8ea?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/16437/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16437   +/-   ##
=======================================
  Coverage    90.4%    90.4%           
=======================================
  Files         161      161           
  Lines       51033    51033           
=======================================
  Hits        46136    46136           
  Misses       4897     4897
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.24% <ø> (ø)` | :arrow_up: |
| #single | `40.17% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.13% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16437?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/16437?src=pr&el=footer). Last update [97ad3fb...fe13093](https://codecov.io/gh/pandas-dev/pandas/pull/16437?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @jorisvandenbossche! Fixed up some of those examples, what do you think now? Thanks @VincentLa 