 DOC: Spellcheck of gotchas.rst (FAQ page)
Minor changes to the documentation, specifically `gotchas.rst`, i.e. the FAQ page.

* Function/method references as links.
* Consistent double backticks ` `` ` around Series, DataFrame.
* Minor rephrasing of a sentence or two, added some missing punctuation.
* Replaced `.. ipython::` with `ipython:: python` twice, as the former is not sufficient to make the iPython code render in the docs.

Reviews/comments very welcome.

