DOC: add warning that utils should only be used internally

doc/developers/index.rst

@@ -71,7 +71,7 @@ repository on
  2. Fork the `project repository
     <http://github.com/scikit-learn/scikit-learn>`__: click on the 'Fork'
     button, at the top, center of the page. This creates a copy of
-    the code on the github server where you can work.
+    the code on the GitHub server where you can work.
 
  3. Clone this copy to your local disk (you need the `git` program to do
     this)::

doc/developers/utilities.rst

@@ -8,8 +8,10 @@ Scikit-learn contains a number of utilities to help with development.  These are
 located in :mod:`sklearn.utils`, and include tools in a number of categories.
 All the following functions and classes are in the module :mod:`sklearn.utils`.
 
-Please note that these utilities are meant to be used internally within
-scikit-learn.  They are not guaranteed to be stable between versions of
+.. warning ::
+
+   These utilities are meant to be used internally within the scikit-learn
+   package.  They are not guaranteed to be stable between versions of
    scikit-learn.  Backports, in particular, will be removed as the scikit-learn
    dependencies evolve.