Docu: _BaseFile class not documented via Sphinx

[buhtz]

The _BaseFile class is missing in the Sphinx HTML docu.

I'm aware that this class is declared as private and Sphinx do not included it because of that.
But that class is mentioned on other components of your docu and in example code. e.g. the use of pofile.append() etc

Include it as private in the docu. Or think about redesign if it really need to be private.