Docblock improvements to the Config library and remove CI_Config::_assign_to_config()

Existance of _assign_to_config() is pointless as this method
consists just of a foreach calling CI_Config::set_item() and
is only called by CodeIgniter.php - moved that foreach() in
there instead.
diff --git a/system/core/CodeIgniter.php b/system/core/CodeIgniter.php
index f3592ea..324b4d8 100644
--- a/system/core/CodeIgniter.php
+++ b/system/core/CodeIgniter.php
@@ -130,9 +130,12 @@
 	$CFG =& load_class('Config', 'core');
 
 	// Do we have any manually set config items in the index.php file?
-	if (isset($assign_to_config))
+	if (isset($assign_to_config) && is_array($assign_to_config))
 	{
-		$CFG->_assign_to_config($assign_to_config);
+		foreach ($assign_to_config as $key => $value)
+		{
+			$CFG->set_item($key, $value);
+		}
 	}
 
 /*