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

@@ -1,3 +1,5 @@
+.. _cpython_diffs:
+
 MicroPython differences from CPython
 ====================================
 

docs/library/index.rst

@@ -1,3 +1,5 @@
+.. _micropython_lib:
+
 MicroPython libraries
 =====================
 

docs/reference/index.rst

@@ -1,13 +1,20 @@
 The MicroPython language
 ========================
 
-MicroPython aims to implement the Python 3.4 standard, and most of
+MicroPython aims to implement the Python 3.4 standard (with selected
-the features of MicroPython are identical to those described by the
+features from later versions) with respect to language syntax, and most
-documentation at
+of the features of MicroPython are identical to those described by the
-`docs.python.org <https://docs.python.org/3.4/reference/index.html>`_.
+"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
+The MicroPython standard library is described in the
-MicroPython are described in the sections here.
+: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