reference/index: Rewrite introduction paragraph to avoid confusion.

The old intro talked about "differences", but there were hardly any
sections describing differences, mostly MicroPython specific features.
On the other hand, we now have real "differences" chapter, though it's
mostly concerned with stdlib differences.

So, try to avoid confusion by changing wording and linking to the other
chapters and contrasting them with what is described in "MicroPython
language".

docs/differences/index_template.txt

+.. _cpython_diffs:
+
 MicroPython differences from CPython
 ====================================
 

docs/library/index.rst

+.. _micropython_lib:
+
 MicroPython libraries
 =====================
 

docs/reference/index.rst

 The MicroPython language
 ========================
 
-MicroPython aims to implement the Python 3.4 standard, and most of
-the features of MicroPython are identical to those described by the
-documentation at
-`docs.python.org <https://docs.python.org/3.4/reference/index.html>`_.
+MicroPython aims to implement the Python 3.4 standard (with selected
+features from later versions) with respect to language syntax, and most
+of the features of MicroPython are identical to those described by the
+"Language Reference" documentation at
+`docs.python.org <https://docs.python.org/3/reference/index.html>`_.
 
-Differences to standard Python as well as additional features of
-MicroPython are described in the sections here.
+The MicroPython standard library is described in the
+:ref:`corresponding chapter <micropython_lib>`. The :ref:`cpython_diffs`
+chapter describes differences between MicroPython and CPython (which
+mostly concern standard library and types, but also some language-level
+features).
+
+This chapter describes features and peculiarities of MicroPython
+implementation and the best practices to use them.
 
 .. toctree::
    :maxdepth: 1