add dropna=False to crosstab example
```
>>> foo = pd.Categorical(['a', 'b'], categories=['a', 'b', 'c'])
>>> bar = pd.Categorical(['d', 'e'], categories=['d', 'e', 'f'])
>>> crosstab(foo, bar)  # 'c' and 'f' are not represented in the data,
    ...                     # but they still will be counted in the output
col_0  d  e  f
row_0
a      1  0  0
b      0  1  0
c      0  0  0
```

The above example code does not produce the output shown because dropna=True is default. Changing crosstab(foo, bar) to crosstab(foo, bar, dropna=False) fixes that and produces the shown output (which is also the expected and correct output).

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

Hello @testvinder! Thanks for updating the PR.



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

##### Comment last updated on June 11, 2018 at 09:29 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21413?src=pr&el=h1) Report
> Merging [#21413](https://codecov.io/gh/pandas-dev/pandas/pull/21413?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/415012f4f38ca0cf41717c51e49bd2349cba09a8?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/21413/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21413?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21413      +/-   ##
==========================================
- Coverage   91.89%   91.89%   -0.01%     
==========================================
  Files         153      153              
  Lines       49596    49596              
==========================================
- Hits        45576    45574       -2     
- Misses       4020     4022       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21413?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/21413/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `97.03% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21413/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.6% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21413?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21413?src=pr&el=footer). Last update [415012f...4d7e36f](https://codecov.io/gh/pandas-dev/pandas/pull/21413?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I completely misread this PR and thought this was a larger change than it actually was.  Sorry! @gfyoung yes, it's really just a tiny change that makes the documentation more clear. @testvinder : I just realized: your PR should be against the `master` branch, not `0.23.x`.  Could you fix that? @gfyoung Done:-) Sorry for the mistake. This is all new to me. @testvinder : I re-applied your commits against `master` because the base-switching was a lot uglier than I was hoping it would be. thanks @testvinder  @jreback just a small note, can you make sure the final commits gets a reasonable message? (normally you should see it in the github merge box), because now it is "Reapply all patches by @testvinder against master" :-) normally i remove all comments - guess missed that one Yeah, it's not about the comments (those go into the commit message body) but the title. But it's a bit strange github did it this wrongly, because normally the title is fine (probably because the commit was rebased and squashed)