Corrected description of the "//" operator

The operator was previously described to truncate the the value when it in fact floors the result of the division (which can be seen in https://github.com/fabpot/Twig/blob/master/lib/Twig/Node/Expression/Binary/FloorDiv.php#L20).
By the old description one would think that ``{{ -20 // 7 }}`` would yield -2 (as in ``(int)(-20/7)``) when it actually returns -3 (as in ``floor(-20/7)``). Floor is not truncate (cast to int).

doc/templates.rst

@@ -617,7 +617,7 @@ Arrays and hashes can be nested:
 .. code-block:: jinja
 
     {% set foo = [1, {"foo": "bar"}] %}
-
+7
 .. tip::
 
     Using double-quoted or single-quoted strings has no impact on performance
@@ -641,9 +641,9 @@ but exists for completeness' sake. The following operators are supported:
 * ``%``: Calculates the remainder of an integer division. ``{{ 11 % 7 }}`` is
   ``4``.
 
-* ``//``: Divides two numbers and returns the truncated integer result. ``{{ 20
-  // 7 }}`` is ``2`` (this is just syntactic sugar for the
-  :doc:`round<filters/round>` filter).
+* ``//``: Divides two numbers and returns the floored integer result. ``{{ 20
+  // 7 }}`` is ``2``, ``{{ -20  // 7 }}`` is ``-3``(this is just syntactic
+  sugar for the :doc:`round<filters/round>` filter).
 
 * ``*``: Multiplies the left operand with the right one. ``{{ 2 * 2 }}`` would
   return ``4``.