Add documentation about type of PYSON

issue3137
review901002

doc/ref/pyson.rst

@@ -74,6 +74,11 @@ value of the statement named by ``value``, if defined in the
 evaluation context, otherwise the ``default`` value (empty
 string by default).  Returns an instance of itself.
 
+.. note::
+
+    The default value determines the type of the statement.
+..
+
 .. class:: Not(value)
 
 A :class:`Not` object represents the PYSON ``Not()``

doc/topics/pyson.rst

@@ -13,9 +13,9 @@ A PYSON parser can easily be implemented in other programming languages.
 So third party softwares do not need to depend on Python to be able to
 fully communicate with the Tryton server.
 
-PYSON is a `deterministic algorithm`_ which will always
-succeed to evaluate statements. There is a default behavior for
-unknown values.
+PYSON is a `deterministic algorithm`_ which will always succeed to evaluate
+statements. There is a default behavior for unknown values. It is statically
+typed and checked on instantiation.
 
 There is also a :ref:`reference documentation of the API <ref-pyson>`.