move the entries about using the Python module into its own (sub) toctree

doc/src/Python_ext.rst

@@ -1,16 +1,19 @@
 Extending the library and Python interface
 ******************************************
 
-As noted above, these Python class methods correspond one-to-one with
+As noted previously, the Python class methods correspond one-to-one with
 the functions in the LAMMPS library interface in ``src/library.cpp`` and
-``library.h``.  This means you can extend the Python wrapper via the
-following steps:
+``library.h``.  This means you can extend the Python wrapper by
+following these steps:
 
 * Add a new interface function to ``src/library.cpp`` and
   ``src/library.h``.
 * Rebuild LAMMPS as a shared library.
 * Add a wrapper method to ``python/lammps.py`` for this interface
   function.
+* Define the corresponding ``argtypes`` list and ``restype``
+  in the ``lammps.__init__()`` function.
+* Re-install the shared library and the python module, if needed
 * You should now be able to invoke the new interface function from a
   Python script.