Update default.rst

Add warning for potentially confusing behaviour and alternative solution.

doc/filters/default.rst

@@ -21,6 +21,15 @@ undefined:
 .. code-block:: twig
 
     {{ var.method(foo|default('foo'))|default('foo') }}
+    
+Using the ``default`` filter on a boolean variable might trigger unexpected behaviour, as
+``false`` is treated as an empty value. Consider using ``??`` instead:
+
+.. code-block:: twig
+
+    {% set foo = false %}
+    {{ foo|default(true) }} {# true #}
+    {{ foo ?? true }} {# false #}
 
 .. note::