DOC: spellchecked io.rst
Spellcheck of the docs, specifically of `io.rst`.
I did not read everything thoroughly, but I believe the suggested changes will help enforce a greater level of consistency in the docs.

* Backticks ` `` ` around Series, DataFrame.
* Added some function references, i.e. `` :meth:`~DataFrame.method` ``.
* Consistency of booleans, typsetting as ``False`` instead of False or *False*.
* Minor rephrasing of sentences, spelling, periods, colons etc.

Cheers!
