Move archive description from TODO to docstring.
Move archive description from TODO to docstring.

This whole class will be refactored, so the TODO is implicit.

xavfernandez/pip,rouge8/pip,xavfernandez/pip,pypa/pip,sbidoul/pip,rouge8/pip,sbidoul/pip,xavfernandez/pip,pfmoore/pip,pradyunsg/pip,pfmoore/pip,pypa/pip,rouge8/pip,pradyunsg/pip