added `ImmutableSandboxedEnvironment`.

--HG--
branch : trunk
diff --git a/docs/sandbox.rst b/docs/sandbox.rst
index a919df8..2b65f9c 100644
--- a/docs/sandbox.rst
+++ b/docs/sandbox.rst
@@ -20,8 +20,12 @@
 .. autoclass:: SandboxedEnvironment([options])
     :members: is_safe_attribute, is_safe_callable
 
+.. autoclass:: ImmutableSandboxedEnvironment([options])
+
 .. autoexception:: SecurityError
 
 .. autofunction:: unsafe
 
 .. autofunction:: is_internal_attribute
+
+.. autofunction:: modifies_builtin_mutable