added missing documentation for the random function

doc/functions/index.rst

@@ -7,6 +7,7 @@ Functions
     range
     cycle
     constant
+    random
     attribute
     block
     parent

doc/functions/random.rst

@@ -0,0 +1,11 @@
+``random``
+==========
+
+.. versionadded:: 1.5
+    The random function was added in Twig 1.5.
+
+The ``random`` function returns a random item from a sequence:
+
+.. code-block:: jinja
+
+    {{ random(['apple', 'orange', 'citrus']) }}