refactored documentation

doc/filters/default.rst

@@ -0,0 +1,18 @@
+``default``
+===========
+
+The ``default`` filter returns the passed default value if the value is
+undefined or empty, otherwise the value of the variable:
+
+.. code-block:: jinja
+
+    {{ var|default('var is not defined') }}
+
+    {{ var.foo|default('foo item on var is not defined') }}
+
+    {{ ''|default('passed var is empty')  }}
+
+.. note::
+
+    Read the documentation for the :doc:`defined<../tests/defined>` and
+    :doc:`empty<../tests/empty>` tests to learn more about their semantics.