Refer local_settings in doc to customize settings

There are couple of places in the docs where it isn't explicitly
mentioned whether to modify settings.py or local_settings.py to
customize settings (this might not be obvious to people who are new
to Horizon and Django).

The changes include:
- In the `Customizing Horizon` guide, under the section 'Modifying
  Existing Dashboards and Panels', explicitly mention local_settings.py
  file to be modified to update ``HORIZON_CONFIG`` setting
- In the QuickStart guide, under the `Setup` section, add a note which
  mentions that to customize any settings use the local_settings.py file
- In the 2013 Havana release notes, under section `Upgrade Information'
  for 'Allowed Hosts', specify only local_settings.py file to add the
  new 'ALLOWED_HOSTS' setting (instead of settings.py or local_settings.py)

Change-Id: I38314cadd8a5a61d1edea26c6cd0d34e713fd66c
Closes-Bug: 1301484

doc/source/quickstart.rst

@@ -34,6 +34,10 @@ Next you will need to setup your Django application config by copying ``openstac
 
     > cp openstack_dashboard/local/local_settings.py.example openstack_dashboard/local/local_settings.py
 
+.. note::
+
+    To add new settings or customize existing settings, modify the ``local_settings.py`` file.
+
 Horizon assumes a single end-point for OpenStack services which defaults to
 the local host (127.0.0.1). If this is not the case change the
 ``OPENSTACK_HOST`` setting in the ``openstack_dashboard/local/local_settings.py`` file, to the actual IP address of the OpenStack end-point Horizon should use.

doc/source/releases/2013_2.rst

@@ -192,7 +192,7 @@ Allowed Hosts
 -------------
 
 For production deployments of Horizon you must add the ``ALLOWED_HOSTS``
-setting to your ``settings.py`` or ``local_settings.py`` file. This setting
+setting to your ``local_settings.py`` file. This setting
 was added in Django 1.5 and is an important security feature. For more
 information on it please consult the ``local_settings.py.example`` file
 or Django's documentation.

doc/source/topics/customizing.rst

@@ -49,9 +49,9 @@ This allows for common site-customization requirements such as:
 * Re-ordering panels within a dashboard or panel group.
 
 To specify the python module containing your modifications, add the key
-``customization_module`` to your ``settings.HORIZON_CONFIG`` dictionary.
-The value should be a string containing the path to your module in dotted
-python path notation. Example::
+``customization_module`` to your ``HORIZON_CONFIG`` dictionary in
+``local_settings.py``. The value should be a string containing the path to your 
+module in dotted python path notation. Example::
 
     HORIZON_CONFIG = {
         "customization_module": "my_project.overrides"