clarify the docs for the top-level kwant namespace

doc/source/reference/kwant.rst

@@ -3,9 +3,9 @@
 
 .. module:: kwant
 
-Along with the various modules documented in this reference, the top-level
-kwant package provides short names for a few widely used objects from
-several modules.
+The top-level kwant package serves as a container for sub-packages and does not
+have any functions of its own.  For convenience, short names are provided for a
+few widely used objects from the sub-packages:
 
 .. currentmodule:: kwant.builder
 

doc/source/reference/kwant.system.rst

-:mod:`kwant.system` -- Low-level interface of tight binding systems
-*******************************************************************
+:mod:`kwant.system` -- Low-level interface of systems
+*****************************************************
 
 .. currentmodule:: kwant.system
 

kwant/system.py

-"""Low-level interface of tight binding systems"""
+"""Low-level interface of systems"""
 
 from __future__ import division
 __all__ = ['System', 'FiniteSystem', 'InfiniteSystem']