[doc] tuning a few things in documentation.

docs/reference/api_util.rst

@@ -0,0 +1,10 @@
+Util API
+========
+
+The Util API, located under the :mod:`bonobo.util` namespace, contains helpers functions and decorators to work with
+and inspect transformations, graphs, and nodes.
+
+.. automodule:: bonobo.util
+    :members:
+    :undoc-members:
+    :show-inheritance:

docs/reference/index.rst

@@ -9,6 +9,7 @@ means that the api is not yet 1.0-proof.
 
     api
     api_config
+    api_util
     commands
     settings
     examples

docs/reference/settings.rst

@@ -3,7 +3,8 @@ Settings & Environment
 
 .. module:: bonobo.settings
 
-All settings that you can find in the :module:`bonobo.settings` module.
+All settings that you can find in the :mod:`bonobo.settings` module. You can override those settings using
+environment variables. For you own settings and configuration values, see the :doc:`/guide/environment` guide.
 
 Debug
 :::::