Activate the Sphinx doctest extension and convert howto/functional to use it.
diff --git a/Doc/conf.py b/Doc/conf.py
index 82d16b8..f6c08c5 100644
--- a/Doc/conf.py
+++ b/Doc/conf.py
@@ -13,7 +13,8 @@
 # General configuration
 # ---------------------
 
-extensions = ['sphinx.ext.refcounting', 'sphinx.ext.coverage']
+extensions = ['sphinx.ext.refcounting', 'sphinx.ext.coverage',
+              'sphinx.ext.doctest']
 templates_path = ['tools/sphinxext']
 
 # General substitutions.