DOC: XportReader not top-level API
@kshedden `XportReader` is not top-level API, so it was erroring in the doc build.

I now removed it from api.rst (the `TextFileReader` what you get back with chunksize for read_csv is also not included), but I can also adapt it to use `.. currentmodule:: pandas.io.sas` so it builds correctly if you want.
