DOC: Spellcheck of categorical.rst and visualization.rst
Minor changes to the documentation, specifically `categorical.rst` and `visualization.rst`:

* Function references as links.
* Backticks ` `` ` around Series, DataFrame.
* Minor rephrasing of sentences, spelling, etc.
* For plots such as lag plots, Andrews plot, Radviz, I added links to Wikipedia entries.

Comments very welcome.
