DOC: fix doc for crosstab with Categorical data input
Crosstab function should set dropna=False to keep the categories which not appear in the data, but in the DOC, it seems to be inconsistent with the description. Two examples for comparison may be better for users to get this point.

https://pandas.pydata.org/docs/dev/user_guide/reshaping.html#cross-tabulations
Thanks for the PR.

Looks like there are some minor fixes:

```
Check that no file in the repo contains trailing whitespaces
##[error]doc/source/user_guide/reshaping.rst:475:categories included in the cross-tabulation while setting ``dropna=False``,  <- trailing whitespaces found
``` Hi, whether I need to close this pull and create another? Thanks, this is my first PR, not very sophisticated about the process. Oh, I find it start automatically after the push step. Thanks! Thanks @GYHHAHA ! Could you please include a screenshot of this part of the docs? See [building the documentation](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#building-the-documentation)

If you're in the `doc` directory, something like this should work:
```bash
python make.py clean
python make.py --single user_guide/reshaping.rst 
``` Hi, this is the screenshot. Is it OK? Thanks! @MarcoGorelli 
![pull_reshaping_doc](https://user-images.githubusercontent.com/41546976/80304841-5157e300-87eb-11ea-8813-1016a3414dd3.png) Hi, this is the screenshot under current status. Thanks for review! @jreback 
![crosstab_cate](https://user-images.githubusercontent.com/41546976/80325176-b43a9000-8866-11ea-9f29-8bcb65b7d191.png)
 lgtm long on green Checks are down, is it ok? Thanks! @jreback  thanks 